perf(storage): remeove erroneous variable
perf(storage): remeove erroneous variable

This commit removes an unused slice that was being built up. Comparting
to the base-line performance yields a slight improvement when deleting
from the cache.

name                                          old time/op    new time/op    delta
Engine_DeletePrefixRange_Cache/exists-24         268ms ± 5%     262ms ± 6%    ~     (p=0.218 n=10+10)
Engine_DeletePrefixRange_Cache/not_exists-24     265ms ± 5%     266ms ± 4%    ~     (p=0.965 n=10+8)

name                                          old alloc/op   new alloc/op   delta
Engine_DeletePrefixRange_Cache/exists-24        64.1MB ± 0%    62.7MB ± 0%  -2.16%  (p=0.000 n=9+9)
Engine_DeletePrefixRange_Cache/not_exists-24    59.2MB ± 0%    59.2MB ± 0%    ~     (p=0.505 n=8+8)

name                                          old allocs/op  new allocs/op  delta
Engine_DeletePrefixRange_Cache/exists-24          711k ± 0%      711k ± 0%  -0.00%  (p=0.000 n=9+9)
Engine_DeletePrefixRange_Cache/not_exists-24      700k ± 0%      700k ± 0%    ~     (p=0.687 n=8+8)

Raw benchmarks using a 24T / 32GB / NVME machine:

goos: linux
goarch: amd64
pkg: github.com/influxdata/influxdb/tsdb/tsm1
BenchmarkEngine_DeletePrefixRange_Cache/exists-24         	      30	 267664312 ns/op	62689106 B/op	  711400 allocs/op
BenchmarkEngine_DeletePrefixRange_Cache/exists-24         	      30	 255017152 ns/op	62688809 B/op	  711398 allocs/op
BenchmarkEngine_DeletePrefixRange_Cache/exists-24         	      30	 258136039 ns/op	62689626 B/op	  711404 allocs/op
BenchmarkEngine_DeletePrefixRange_Cache/exists-24         	      30	 273982453 ns/op	62688325 B/op	  711395 allocs/op
BenchmarkEngine_DeletePrefixRange_Cache/exists-24         	      30	 252670795 ns/op	62688704 B/op	  711397 allocs/op
BenchmarkEngine_DeletePrefixRange_Cache/exists-24         	      30	 277700985 ns/op	61801204 B/op	  702520 allocs/op
BenchmarkEngine_DeletePrefixRange_Cache/exists-24         	      30	 272353886 ns/op	62688767 B/op	  711403 allocs/op
BenchmarkEngine_DeletePrefixRange_Cache/exists-24         	      30	 258717468 ns/op	62689461 B/op	  711408 allocs/op
BenchmarkEngine_DeletePrefixRange_Cache/exists-24         	      30	 252909070 ns/op	62688949 B/op	  711404 allocs/op
BenchmarkEngine_DeletePrefixRange_Cache/exists-24         	      30	 255436837 ns/op	62689712 B/op	  711409 allocs/op
BenchmarkEngine_DeletePrefixRange_Cache/not_exists-24     	      30	 241173429 ns/op	59202122 B/op	  700036 allocs/op
BenchmarkEngine_DeletePrefixRange_Cache/not_exists-24     	      30	 247961098 ns/op	60507541 B/op	  714102 allocs/op
BenchmarkEngine_DeletePrefixRange_Cache/not_exists-24     	      20	 263380230 ns/op	59202750 B/op	  700044 allocs/op
BenchmarkEngine_DeletePrefixRange_Cache/not_exists-24     	      30	 266035285 ns/op	59202758 B/op	  700043 allocs/op
BenchmarkEngine_DeletePrefixRange_Cache/not_exists-24     	      30	 266512878 ns/op	59202759 B/op	  700044 allocs/op
BenchmarkEngine_DeletePrefixRange_Cache/not_exists-24     	      30	 262065769 ns/op	59202726 B/op	  700043 allocs/op
BenchmarkEngine_DeletePrefixRange_Cache/not_exists-24     	      30	 270485538 ns/op	59202733 B/op	  700043 allocs/op
BenchmarkEngine_DeletePrefixRange_Cache/not_exists-24     	      30	 263355678 ns/op	62562757 B/op	  727794 allocs/op
BenchmarkEngine_DeletePrefixRange_Cache/not_exists-24     	      30	 260440337 ns/op	59203324 B/op	  700050 allocs/op
BenchmarkEngine_DeletePrefixRange_Cache/not_exists-24     	      30	 276122362 ns/op	59203316 B/op	  700050 allocs/op
PASS
ok  	github.com/influxdata/influxdb/tsdb/tsm1	259.435s

mark-rushakoff/influxdb,influxdb/influxdb,influxdata/influxdb,nooproblem/influxdb,mark-rushakoff/influxdb,nooproblem/influxdb,influxdb/influxdb,li-ang/influxdb,mark-rushakoff/influxdb,nooproblem/influxdb,influxdb/influxdb,influxdb/influxdb,mark-rushakoff/influxdb,mark-rushakoff/influxdb,nooproblem/influxdb,influxdb/influxdb,influxdata/influxdb,mark-rushakoff/influxdb,li-ang/influxdb,nooproblem/influxdb,influxdb/influxdb