drivers: can: mcp2515: Remove MCP2515 specifics

CAN_MCP2515_MAX_FILTER is not needed anymore and was probably just a
misunderstanding. Aligned it with the other CAN drivers.
This also was the last difference in the mcp2515 specific config for the
can sample, so that can be deleted as well.

Signed-off-by: Karsten Koenig <karsten.koenig.030@gmail.com>