[build] Remove #pragma diagnostic for boost

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

src/mbgl/util/http_header.cpp

@@ -2,20 +2,9 @@
 
 #include <mbgl/util/string.hpp>
 
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wunknown-pragmas"
-#pragma GCC diagnostic ignored "-Wunused-parameter"
-#pragma GCC diagnostic ignored "-Wshadow"
-#pragma clang diagnostic push
-
-#pragma clang diagnostic ignored "-Wshorten-64-to-32"
-#pragma clang diagnostic ignored "-Wunknown-warning-option"
-#pragma clang diagnostic ignored "-Wtautological-constant-compare"
 #include <boost/spirit/include/qi.hpp>
 #include <boost/spirit/include/phoenix_core.hpp>
 #include <boost/spirit/include/phoenix_operator.hpp>
-#pragma clang diagnostic pop
-#pragma GCC diagnostic pop
 
 namespace mbgl {
 namespace http {