Remove API_RATE_LIMIT variable since it's wrong anyway

The api_rate_limit config option in nova has defaulted to False since
Havana per commit cfdc59860ef1482e3b064f90f719ec9e484590ce. This change
removes the variable and code that's used to set it in nova.conf since
if we left it in we'd change the default to False to match the code, and
we'd be breaking anyone that is relying on this to be True anyway.

Note that this variable exists in devstack essentially for Tempest so we
can set it in devstack-gate, which is kind of a moot point now given the
code defaults to it anyway.

Change I21dbd72f9959a4e3d62c36831df03b351a51172e removes it from
devstack-gate.

Change-Id: Ib0538bdd23b17e519b9c917018ccc9fa8c6425c5
Merge "Remove API_RATE_LIMIT variable since it's wrong anyway"