Remove ThreadContext::SymbolTable.

The symbol table is now available directly from the SkSL::Context.
We don't need a dedicated accessor for it anymore (and can avoid
a trip into thread-local storage). We can also avoid passing a
reference to the current symbol table to functions that already
accept a Context.

Change-Id: Ic58794f8c29afd8d6eb994ce411ffacbfbb7a68b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/693258
Auto-Submit: John Stiles <johnstiles@google.com>
Reviewed-by: Arman Uguray <armansito@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
26 files changed