LU-10826 ptlrpc: fix test_req_buffer_pressure behavior

LU-10826 ptlrpc: fix test_req_buffer_pressure behavior

In 2nd patch for LU-9372, to allow limiting number of rqbd-buffers,
a wrong and unnecessary test had been added to enhance
test_req_buffer_pressure feature.
This patch fixes this issue by removing such test.

Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: I8f1298fabbbc4e0d92078dcf49192ff4f0fbc907
Reviewed-on: https://review.whamcloud.com/31690
Tested-by: Jenkins
Reviewed-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Li Dongyang <dongyangli@ddn.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>