[NaCl SDK] Remove old .gitignore file

The contents of this file seems to be all legacy
stuff.  These days all the build products end up in
the 'out' directory.

Review URL: https://codereview.chromium.org/835233004

Cr-Commit-Position: refs/heads/master@{#311367}

native_client_sdk/src/.gitignore

-# This is a standard .gitignore file, used to exclude certain files
-# from a 'git status' command, or similar
-
-# Ignore these filename patterns
-*.cmd
-*.gz
-*.nexe
-*.nmf
-*.o
-*.pyc
-*.pyo
-
-# Ignore all directories than contain these names
-toolchain/
-
-# Ignore these particular directories (relative to src directory)
-/build_tools/NSIS/
-/build_tools/packages/
-/build_tools/sdk_install_name.nsh
-/build_tools/sdk_section.nsh
-/build_tools/tests/apply_patch_test_archive/
-/build_tools/toolchain_archives/
-/chrome_binaries/
-/debugger/NativeClientVSX/
-/debugger/nacl-gdb_server/x64/
-/experimental/webgtt/tests/nacltest/*.cc
-/staging/