thrift: drop obtaining incorrect permits
thrift: drop obtaining incorrect permits

The thrift layer started partially having admission control
after commit ef1de114f09f84d58038614858608ec3ffca07aa,
but code inspection suggests that it might cause use-after-free
in a few cases, when a permit is obtained more than once per
handling - due to the fact that some functions tail-called other
functions, which also obtain a permit.
These extraneous permits are not taken anyore.

Tests: "please trust me" + cassandra-stress in thrift mode
Message-Id: <fec200cc08158e2f43ce789af9ad9ce44b08f2d9@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla