powerpc/dt_cpu_ftrs: Remove unused macro ISA_V2_07B

Macro ISA_V2_07B is defined but not used anywhere else in the code.

Signed-off-by: Murilo Opsfelder Araujo <muriloo@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200610215114.167544-2-muriloo@linux.ibm.com