arm: mach-k3: j721e: Cleanup MAIN R5 boot code from R5 SPL

The common J7 specific start_non_linux_remote_cores() override function
implements the logic to load and boot the Main R5FSS Core0 from R5 SPL.
This won't be supported any more for either J721E or J7200 after the R5
SPL rearchitecture for the System Firmware split into TI Foundation
Security (TIFS) and Device Management (DM) firmwares. So, cleanup the
corresponding code and the related SPL env variables.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Link: https://lore.kernel.org/r/20210726211311.5977-3-s-anna@ti.com