Update the following functions to their 1.2 core names:
 * vkCreateRenderPass2KHR   --> vkCreateRenderPass2
 * vkCmdBeginRenderPass2KHR --> vkCmdBeginRenderPass2
 * vkCmdEndRenderPass2KHR   --> vkCmdEndRenderPass2
 * vkCmdNextSubpass2KHR     --> vkCmdNextSubpass2

Also remove redundant code in vkCreateRenderPass and
vkCreateRenderPass2.

Bug: b/169593400
Change-Id: I8a5ac34e694fa29796c132fb2be0d5a06fad2912
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/48988
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Sean Risser <srisser@google.com>