Remove [[maybe_unused]] workaround in MetalCodeGen.

This effectively rolls back http://review.skia.org/538045.

Change-Id: I9c457a6c9c9ba98d4c2e3a6af03c5dd80c76beb9
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/540082
Auto-Submit: John Stiles <johnstiles@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>

src/sksl/codegen/SkSLMetalCodeGenerator.cpp

@@ -2148,16 +2148,6 @@ void MetalCodeGenerator::writeName(std::string_view name) {
 }
 
 void MetalCodeGenerator::writeVarDeclaration(const VarDeclaration& varDecl) {
-    // The graphite shaders include functions with `switch` statements using named constant labels.
-    // When called with known values, the switch is eliminated. This leaves unused variables like:
-    //   const int kFoo = ...;
-    // All of these happen to be `const int`. As a hack, mark any `const int` as maybe_unused, to
-    // suppress warnings from the metal compiler when validation is enabled.
-    if ((varDecl.var().modifiers().fFlags & Modifiers::kConst_Flag) &&
-        varDecl.var().type().matches(*fContext.fTypes.fInt)) {
-        this->write("[[maybe_unused]] ");
-    }
-
     this->writeModifiers(varDecl.var().modifiers());
     this->writeType(varDecl.var().type());
     this->write(" ");