[SYCL][NFC] Remove outdated .gitignore file for SYCL library

[bader]

We used to generate two header files to source directory, so .gitignore
file protected developers from committing auto-generated files.
f3b5fad
changed the directory for emitted files from source to build. Now we
don't need this .gitignore file anymore.

[bader]

For fellow DPC++ developers
I cloned the source directory quite a long time ago and this .gitignore file was hiding two auto-generated files in the source directory for months. I noticed that after sycl/test/extensions/macro.cpp failed on clean HEAD of the sycl branch. The test checks SYCL_EXT_ONEAPI_BACKEND_LEVEL_ZERO is equal to 5, but it turned out to be 3 on my system. It looks like the compiler adds include path to the DPC++ headers from source directory before the include path in build directory and pick up the outdated feature_test.hpp.
The recipe for this issue is to remove auto-generated headers from /sycl/* directory.