Fixed memory leak associated with TLS.

We used to allocate thread-local memory on each compile.
If the compile did not happen on the same thread as ShInitialize,
we leaked the thread-local memory.

It turns out that there is no need to allocate any thread-local
memory. This patch cleans up all the unnecessary junk around TLS.

BUG=chromium:181691

Change-Id: I4b67ab23dc856d93424ae51ebf8aaf8966b732e4
Reviewed-on: https://swiftshader-review.googlesource.com/1361
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>