Revert "main: _mesa_cube_level_complete checks NumLayers."

Revert "main: _mesa_cube_level_complete checks NumLayers."

This reverts commit 1ee000a0b6737d6c140d4f07b6044908b8ebfdc7.
Failures with the GLES3 conformance suite and Synmark2 OGLHdrBloom revealed
that this commit was in error.

Extensive testing with Piglit prior to patch review and upstreaming did not
reveal this problem because, in the few Piglit tests that test for cube
completeness, NumLayers = 6.  This is because all of the existing tests use
TextureStorage to initialize the texture, which sets NumLayers.

A new Piglit test has been sent to the mailing list that reproduces the bug
related to this patch ("texturing: Testing
glGenerateMipmap(GL_TEXTURE_CUBE_MAP) without glTexStorage2D").

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>