Web: Selectively remove a few key features, using #ifndef GLSLANG_WEB

Save about 100K.

N.B.: This is done by eliminating a function call, at a high level,
not by #ifdef'ing a bunch of code.

Also, removed no longer needed *_EXTENSION #ifdef in the code not
needed by GLSLANG_WEB.

SPIRV/GlslangToSpv.cpp

@@ -1592,9 +1592,11 @@ void TGlslangToSpvTraverser::finishSpv()
     for (auto it = iOSet.cbegin(); it != iOSet.cend(); ++it)
         entryPoint->addIdOperand(*it);
 
+#ifndef GLSLANG_WEB
     // Add capabilities, extensions, remove unneeded decorations, etc., 
     // based on the resulting SPIR-V.
     builder.postProcess();
+#endif
 }
 
 // Write the SPV into 'out'.