tor-keymgr: Abolish arti_path_prefix()

The following discussion from !1784 (merged) should be addressed:

• @Diziet started a discussion: (+1 comment)

  I'm still unsure of the need for this function. Its difference from arti_pattern is that it treats the path denotators differently from the non-path denontators but I don't see why that's useful.

  I looked for callers and found only the call in arti_pattern and some tests.

  Also the name "prefix" is wrong, because the word is defined differently in the doc ccomment for the KeySpecifierDefault macro.