DirectX9 Backend Removal: Remove shader model 3 code paths

RendererD3D::getMajorShaderModel() now always returns >= 4, since the
D3D9 backend and its Feature Level 9_3 support are gone. That makes
every shader-model-3 code path in the D3D11 shader generator dead.
Remove them, along with the FL9_3-only features that depended on them.

Removed:
- All getMajorShaderModel() < 4 / == 3 / <= 3 branches in DynamicHLSL,
  ProgramD3D and ProgramExecutableD3D. The GetVaryingSemantic() helper
  is now a constant ("TEXCOORD"), so it and ProgramD3DMetadata's cached
  shader model(getRendererMajorShaderModel / mRendererMajorShaderModel)
  are deleted, and the now-unused `renderer` / `programUsesPointSize`
  parameters are dropped from the affected helpers.

- The instanced point-sprite emulation (StateManager11's
  updateVertexOffsetsForPointSpritesEmulation() and
  GetReservedBufferCount()), which was the FL9_3 fallback for hardware
  without geometry shaders. Point sprites on FL10+ are still emulated
  with a geometry shader.

- The dead vertex-shader driver constants dx_ViewAdjust (D3D9 large
  viewport emulation) and dx_ViewCoords (only referenced by the
  instanced point-sprite path). The vertex DriverConstants cbuffer and
  the matching CPU-side Vertex struct shrink by two registers; the
  remaining packoffsets and the sampler-metadata register are shifted
  to match. The pixel-shader dx_ViewCoords is kept, as the point-sprite
  geometry shader reads it.

Bug: chromium:40874754
Change-Id: I6109d57193f5afdf507cb1eb03e67d3eac79d619
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/8032532
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Rafael Cintron <rafael.cintron@microsoft.com>