[CMAKE] Remove useless remove_compile_flag in leveldb

Summary:
LevelDB adds a -Wno-strict-prototypes then removes
-Wstrict-prototypes. This is redundant and does not work anymore since
it tries to remove the flag from the CFLAGS which are no longer used
for this purpose since D4947.

Test Plan:

cmake -GNinja .. -DCMAKE_C_FLAGS="-Werror=strict-prototypes"
ninja check-leveldb

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4958