tools: git hooks: drop "submitted by" from commit template

With the switch to git, we should strive to properly attribute every
commit appropriately with the metadata that's provided to do so. In this
case, the submitter should be recorded via the author metadata. Committing
an arbitrary patch, one can set it as such:

git commit --author="John Smith <smith@example.com>"

Reviewed-by: emaste
Differential-Revision: https://reviews.freebsd.org/D28069