Clean up documentation that references deprecated 'git peek-remote'.

Clean up documentation that references deprecated 'git peek-remote'.

Now that 'git peek-remote' is deprecated and only an alias for
'git ls-remote', it should not be referenced from other manual pages.

This also removes the description of the --exec option, which is no
longer present.

Signed-off-by: Johannes Sixt <redacted>
Signed-off-by: Junio C Hamano <redacted>