This effectively reverts the following CL:
Vulkan: Don't detach separable shaders in Program::detachShader()
https://chromium-review.googlesource.com/c/angle/angle/+/2084399

Bug: angleproject:3570
Bug: b/182409935
Test: ProgramPipelineTest31.DetachAndModifyShader
Change-Id: Id69f732d1d4f3eea70b2de8c008452878eaf1682
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2776267
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>