Inactive vertex attributes are harmless to remove.  Between two
consecutive stages, the input varyings must be a subset of the previous
stage's output varyings.  This means removing inactive input varyings is
also harmless.

Removing inactive output varyings is not possible though.  GLSL allows a
varying to not be written by the previous stage even if it's used in the
current stage (values will be undefined, but it's not an error).  This
means that an inactive output varying may still need to exist as part of
the shader interface in case the following stage has that varying as
input (and is active).

Bug: angleproject:3394
Change-Id: I7302973d2b8356d9f54a66f8259c32f245a99904
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2009986Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Tim Van Patten <timvp@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>