defconfig: sdm670: Disable DEBUG_OBJECTS_RCU_HEAD to save memory

DEBUG_OBJECTS_RCU_HEAD has significant memory usage. In
cases of stress testing, where RCU gets excessive usage,
these debug objects increases memory pressure and yields
false positive memory issues. Disable by default.

Change-Id: I14c0b688727c7ee1d391865028db2c887a8f0863
Signed-off-by: Chintan Pandya <cpandya@codeaurora.org>