folly: remove warning->error promotion from cmake build

Summary:
Running down various warning/error issues is proving
to be a tiresome game of wackamole.   Here's the latest issue:

```
folly/container/test/F14TestUtil.h:400:16: error: ?void* mem
      std::memcpy(p, &n, sizeof(std::size_t));
      ~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /opt/rh/devtoolset-8/root/usr/include/c++/8/bits/stl_algobase.h:64,
                  from /opt/rh/devtoolset-8/root/usr/include/c++/8/bits/char_traits.h:39,
                  from /opt/rh/devtoolset-8/root/usr/include/c++/8/string:40,
                  from /opt/rh/devtoolset-8/root/usr/include/c++/8/stdexcept:39,
                  from /data/users/wez/scratch/dataZusersZwezZfbsource/fbcode_builder_getdeps/shipit/folly/folly/container/F14Map.h:30,
                  from /data/users/wez/scratch/dataZusersZwezZfbsource/fbcode_builder_getdeps/shipit/folly/folly/container/test/F14MapTest.cpp:17
/opt/rh/devtoolset-8/root/usr/include/c++/8/bits/stl_pair.h:198:12: note: ?struct std::pair<const std::basic_string<char>, int>? declared here
      struct pair
            ^~~~
```

The targeted solution is to use `-Wno-class-memaccess` as we do with our
internal builds, but that is a gcc-specific option that fails when used with
the apple clang compiler.

The simplest option is just to disable warning->error promotion in
the cmake build, as it feels like a losing battle to keep chasing
the different combinations/permutations.

Reviewed By: yfeldblum

Differential Revision: D14702575

fbshipit-source-id: d8daa6c9588e7c84c991a0ef86baf700bd32fe8c

CMake/FollyCompilerUnix.cmake

@@ -34,7 +34,6 @@ function(apply_folly_compile_options_to_target THETARGET)
       -std=${CXX_STD}
       -finput-charset=UTF-8
       -fsigned-char
-      -Werror
       -Wall
       -Wno-deprecated
       -Wno-deprecated-declarations