Remove gitleaks scanning

Ref:
#21570 (comment)

This tool is really intended/best used from git pre-commit on developers
local machines, to prevent addition of secret leaks.  When used as a
check against PRs, it tends to turn up more false-positives than helpful
warnings.  There's no good way to fix this, and maintaining the scanner
is an additional burden.  Rather than continue struggling to improve/fix
the situation, let's just remove the tool entirely.

Signed-off-by: Chris Evich <[email protected]>