glsl: Remove unused ast copy constructors.
glsl: Remove unused ast copy constructors.

These were added in commits a760c738 and 43757135 to be used in
implementing C-style aggregate initializers (commit 1b0d6aef). Paul
rewrote that code in commit 0da1a2cc to use GLSL types, rather than
AST types, leaving these copy constructors unused.

Tested by making them private and providing no definition.

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler