Also modify or remove some tests to sync up with the expected behavior
stated in spec.

Related to https://github.com/KhronosGroup/WebGL/pull/2780
 If any draw buffer with an attachment does not have a defined fragment shader output,
 draws generate INVALID_OPERATION.

Also remove Framebuffer masking for inactive outputs.

This workaround is no longer necessary as the WebGL spec has changed.
It also was never fully working and had bugs with certain orders of
calls.

Bug: angleproject:2872
Bug: chromium:927908
Bug: chromium:943538
Change-Id: I73715a6ab851ae3db7096f49ea0a9fdd6f576703
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1530018
Commit-Queue: Shrek Shao <shrekshao@google.com>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>