[build] Remove #pragma diagnostic for boost

Boost is now included as a system header and the compiler
should ignore it when issuing warnings.

platform/default/utf.cpp

@@ -1,16 +1,7 @@
 #include <mbgl/util/utf.hpp>
 
-#if defined(__GNUC__) && __GNUC__ >= 7
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wimplicit-fallthrough"
-#endif // defined(__GNUC__) && __GNUC__ >= 7
-
 #include <boost/locale/encoding_utf.hpp>
 
-#if defined(__GNUC__) && __GNUC__ >= 7
-#pragma GCC diagnostic pop
-#endif // defined(__GNUC__) && __GNUC__ >= 7
-
 namespace mbgl {
 namespace util {