LU-13555 build: Map mainline kernel on rhel to rhel

LU-13555 build: Map mainline kernel on rhel to rhel

Mainline kernel builds on RHEL can map to RHEL directly and
the MAINLINE_KERNEL abstraction is not needed and can be
removed.

Test-Parameters: trivial
HPE-bug-id: LUS-7689
Signed-off-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Change-Id: I80ba0477c405b9d0f12b4d472f244bb9b15999ff
Reviewed-on: https://review.whamcloud.com/38594
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Petros Koutoupis <petros.koutoupis@hpe.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>