[libc++][test] Remove disable_missing_braces_warning.h from tests

Several tests include `disable_missing_braces_warning.h` but do not need
to. Remove the include.

Inspired from discussion at https://reviews.llvm.org/D109668

Reviewed By: ldionne, #libc, Mordante

Differential Revision: https://reviews.llvm.org/D109711