.clang-format

cleanup: remove pointless r-prefixes on single-quoted strings

This is the promised second step on single-quoted strings. These had
existed because our source transformer didn't turn r'' into b'', so we
had tagged some strings as r-strings to get "native" strings on both
Pythons. Now that the transformer is gone, we can dispense with this
nonsense.

Methodology:

I ran

    hg locate 'set:added() or modified() or clean()' | egrep '.*\.py$'  | xargs egrep --color=never -n  -- \[\^b\]\[\^a-z\]r\'\[\^\'\\\\\]\*\'\[\^\'\

in an emacs grep-mode buffer, and then used a keyboard macro to
iterate over the results and remove the r prefix as needed.

# skip-blame removing unneeded r prefixes left over from Python 3 migration.

Differential Revision: https://phab.mercurial-scm.org/D7306

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  - Regex:           '^<'
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  - Regex:           '^"'
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