Remove unnecessary flag from `grep` call

The `-P` flag for the `grep` command used by the whitespace linter was
unnecessary, as Ruby was expanding the "\t" as a tab character anyway.

Removing this flag also makes this lint more portable, as the -P flag is
GNU-grep-specific.

Change-Id: I5d5dc4dad91ecadc05585447ee8df167ae232adf
Reviewed-on: https://gerrit.causes.com/17411
Reviewed-by: Aiden Scandella <aiden@causes.com>
Reviewed-by: Greg Hurrell <greg@causes.com>
Tested-by: Shane da Silva <shane@causes.com>