Shaders: what in the name of fuck.

That feeling when you lose three hours debugging STRANGE shader compiler
issues that happen only on ES, seeing stuff like "unexpected HASH_TOKEN
at line 140" or "unterminated ifdef" on just any compiler you try, and
then you spot THIS. FFS.

Apparently this is how I was porting shaders in 2013, but not all, I was
mostly sane, wrapping things in a nice ifdef EXPLICIT_UNIFORM_LOCATION,
except this one case in b9a72bd3d1 where I
temporarily went full retard. No idea why.

src/Magnum/Shaders/MeshVisualizer.frag

@@ -32,10 +32,6 @@
 #define const
 #endif
 
-#ifndef EXPLICIT_UNIFORM_LOCATION
-#define layout(arg)
-#endif
-
 #if defined(WIREFRAME_RENDERING) && defined(GL_ES) && __VERSION__ < 300
 #extension GL_OES_standard_derivatives : enable
 #endif