LU-17520 tests: change DEBUG_SIZE logic

LU-17520 tests: change DEBUG_SIZE logic

Don't set DEBUG_SIZE to 2MB*CPU_num. This way
lustre debug buffer could be just 4MB on a system
with 2 CPUs, despite 3GB RAM. It is the reason
why often time period in debug logs doesn't scope
the reason of failure(sometimes depending on debug
level logs it may store just several seconds). When
DEBUG_SIZE is not set, debug_mb would be calculated
inside libcfs based on RAM and CPU number.

Test-Parameters: trivial
Signed-off-by: Sergey Cheremencev <scherementsev@ddn.com>
Change-Id: Iacccc625ec6564c982c75172561c8c3e4114e4b7
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/53988
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Alexander Zarochentsev <alexander.zarochentsev@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>