glsl: revert "glsl: atomic counters can be declared as buffer-qualified variables"

glsl: revert "glsl: atomic counters can be declared as buffer-qualified variables"

This reverts commit 586142658e2927a68c.

The specs are not explicit about any restrictions related to the types allowed
on buffer variables, however, the description of opaque types (like atomic
counters) is in conclict with the purpose of buffer variables:

"The opaque types declare variables that are effectively opaque
 handles to other objects. These objects are
 accessed through built-in functions, not through direct reading or
 writing of the declared variable.
 (...)
 Opaque variables cannot be treated as l-values;(...)"

Also, Mesa is already disallowing opaque types in interface blocks anyway, so
that commit was not really achieving anything.

Reviewed-by: Francisco Jerez <currojerez@riseup.net>