Revert "Introduce a cmake module to figure out whether we need to link with libatomic."

This reverts commit r260235. It breaks LLVM's bootstrap when building
with a -gcc-toolchain and the system's gcc installation does not provide
the libatomic library and its headers. We should check whether
LIBCXX_GCC_TOOLCHAIN is set and adjust the flags accordingly.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@260323 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/test/target_info.py

@@ -172,9 +172,6 @@ class LinuxLocalTI(DefaultTargetInfo):
             flags += ['-lunwind', '-ldl']
         else:
             flags += ['-lgcc_s', '-lgcc']
-        use_libatomic = self.full_config.get_lit_bool('use_libatomic', False)
-        if use_libatomic:
-            flags += ['-latomic']
         san = self.full_config.get_lit_conf('use_sanitizer', '').strip()
         if san:
             # The libraries and their order are taken from the