REThreaded: convert gen- and delete-Textures to no-ops

Bug: 199501005
Test: make
Test: perfetto trace

These functions are not implemented by SkiaGLRenderEngine, so blocking
on them does not serve any purpose. Skip them unless we're running the
THREADED (implicitly GLES) version.

In a local perfetto trace, this reduces the time for these methods from
around 100 microseconds to around 1 microsecond.

Change-Id: I86945d5a1e565db423c181bc828c3f168301784b