Remove CHECK around duplicate FLAG lookup.
Remove CHECK around duplicate FLAG lookup.

It causes an asan initialization-order-fiasco in trying to read the
names of other globally constructed data:

==21449==ERROR: AddressSanitizer: initialization-order-fiasco on address 0x7f6f297bc5e8 at pc 0x7f6f26b332a7 bp 0x7ffd479f8cb0 sp 0x7ffd479f8ca8
READ of size 8 at 0x7f6f297bc5e8 thread T0
    #0 0x7f6f26b332a6 in name
webrtc/base/flags.h:83:38
    #1 0x7f6f26b332a6 in Lookup
webrtc/base/flags.cc:133
    #2 0x7f6f26b332a6 in rtc::FlagList::Register(rtc::Flag*)
webrtc/base/flags.cc:260
    #3 0x7f6f2529972b in __cxx_global_var_init.1

BUG=

Review-Url: https://codereview.webrtc.org/2110963004
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#13479}

TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc