Remove an unused compiler flag check

The check for -Wno-missing-declarations silenced warnings in the
formerly vendored GoogleTest sources. Its result has not been used since
these sources were replaced by the GoogleTest submodule.

Drop the check.

Change-Id: Ia86c4de27216decef41de7ff3ea8b58160759e5c

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 708baea..8d30d7f 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -190,10 +190,6 @@
   REQUIRED RECOMMENDED OPTIONAL)
 
 check_cxx_compiler_flag(/bigobj HAVE_BIGOBJ)
-# Ceres C++ compiler flags can be too strict for an external library code
-# which we do not maintain.
-check_cxx_compiler_flag("-Wno-missing-declarations"
-  CHECK_CXX_FLAG_Wno_missing_declarations)
 # Older versions of Clang (<= 2.9) do not support the 'return-type-c-linkage'
 # option, so check for its presence before adding it to the default flags set.
 check_cxx_compiler_flag("-Wno-return-type-c-linkage"