[libcxx] Fix .gitignore to not exclude test directories

We have several test directories whose names end in .spec. Ensure we
don't accidentally ignore them by removing a section of the .gitignore
that's irrelevant for libc++.

Differential Revision: https://reviews.llvm.org/D69195

libcxx/.gitignore

@@ -22,12 +22,6 @@ var/
 .installed.cfg
 *.egg
 
-# PyInstaller
-#  Usually these files are written by a python script from a template
-#  before PyInstaller builds the exe, so as to inject date/other infos into it.
-*.manifest
-*.spec
-
 # Installer logs
 pip-log.txt
 pip-delete-this-directory.txt