qa: Remove multiple gateway conf from nvmeof thrasher

Remove these files:
- qa/suites/nvmeof/thrash/gateway-initiator-setup/2-subsys-8-namespace.yaml
- qa/suites/nvmeof/thrash/gateway-initiator-setup/120-subsys-8-namespace.yaml

This is because now we can use overrides to change
gateway configuration.

Signed-off-by: Vallari Agrawal <[email protected]>

Author: VallariAg