Bug 1539241 - Remove MOZ_IS_MSVC for being unused and misleadingly named r=froydnj

Differential Revision: https://phabricator.services.mozilla.com/D24954

--HG--
extra : moz-landing-system : lando

mfbt/Compiler.h

@@ -10,7 +10,6 @@
 #define mozilla_Compiler_h
 
 #define MOZ_IS_GCC 0
-#define MOZ_IS_MSVC 0
 
 #if !defined(__clang__) && defined(__GNUC__)
 
@@ -30,11 +29,6 @@
 #    error "mfbt (and Gecko) require at least gcc 6.1 to build."
 #  endif
 
-#elif defined(_MSC_VER)
-
-#  undef MOZ_IS_MSVC
-#  define MOZ_IS_MSVC 1
-
 #endif
 
 /*