Unify functions for multi-buffer in preparation for detection negated content + absent buffer 2224 v12.12

[catenacyber]

Link to redmine ticket: https://redmine.openinfosecfoundation.org/issues/6575

Describe changes:

• detect: unify multi-buffer code

#11070 rebased after merge of #11003 😢 And also a few more unused struct defintions removals

This is required for #10334