Drop unused DLL_RUNTIME support on Windows

We link C library statically whenever possible.
The flag is currently unused see [this codesearch query](https://a.yandex-team.ru/search?search=%255CbDLL_RUNTIME%255Cb%2C%2C%2Carcadia%2C%2C200).
5d6c96de31fce223e268f32ea1f2513a6aa682eb

build/conf/compilers/msvc_compiler.conf

@@ -28,17 +28,10 @@ when ($IDE_MSVS == "yes") {
     DEBUG_INFO_FLAGS=/Zi /FS
 }
 
+# For the flags regarding runtime library selection, see
 # https://msdn.microsoft.com/en-us/library/abx4dbyh.aspx
-when ($DLL_RUNTIME == "yes") {
-    _FLAGS_DEBUG_RUNTIME=/MDd
-    _FLAGS_RELEASE_RUNTIME=/MD
-}
-otherwise {
-    _FLAGS_DEBUG_RUNTIME=/MTd
-    _FLAGS_RELEASE_RUNTIME=/MT
-}
-CFLAGS_DEBUG   = /Od /Ob0 /Oi /D_DEBUG $_FLAGS_DEBUG_RUNTIME
-CFLAGS_RELEASE = /O2 /Ob2 /Oi /DNDEBUG $_FLAGS_RELEASE_RUNTIME
+CFLAGS_DEBUG   = /Od /Ob0 /Oi /D_DEBUG /MTd
+CFLAGS_RELEASE = /O2 /Ob2 /Oi /DNDEBUG /MT
 
 MASMFLAGS=
 _MASM_IO=/nologo /c /Fo${output;suf=${OBJECT_SUF}:SRC} ${input:SRC}