[SkSL][WGSL] Remove polyfillInfo null check assert

Remove this assert as it would trigger for non polyfillable matrix or
array variable types defined by the logic at
https://source.chromium.org/chromium/chromium/src/+/main:third_party/skia/src/sksl/codegen/SkSLWGSLCodeGenerator.cpp;l=4789;drc=dd26c181cbe834ae5d1cdc37677ce03df064a1bc.

Change-Id: I49eca07761d1aa083c06f07b67194929a0fe799e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1293876
Commit-Queue: Nathan Sanchez <nathanasanchez@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>