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.

glslang/MachineIndependent/ParseHelper.cpp

@@ -4737,9 +4737,11 @@ void TParseContext::finish()
     if (parsingBuiltins)
         return;
 
+#ifndef GLSLANG_WEB
     // Check on array indexes for ES 2.0 (version 100) limitations.
     for (size_t i = 0; i < needsIndexLimitationChecking.size(); ++i)
         constantIndexExpressionCheck(needsIndexLimitationChecking[i]);
+#endif
 
     // Check for stages that are enabled by extension.
     // Can't do this at the beginning, it is chicken and egg to add a stage by