Revert "glsl: geom shader max_vertices layout must match."

Revert "glsl: geom shader max_vertices layout must match."

This reverts commit 4c863993780a11cea6f88fa0682796bee5794042.

The commit was erroneous because the ast_layout_expression class was
created to hold a list of values for a layout-qualifier-name which is
allowed to appear in more than one expression in the same
shader/program but not to hold different values.

In other words, the list is used for an after check that all the
declared values for a layout-qualifier-name are consistent.

Therefore, the values stored must match always, not just for
"max_vertices" or any other eventual layout-qualifier-name.

Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>
Signed-off-by: Andres Gomez <agomez@igalia.com>