Stop treating projectile-consult.el as a standalone package

[bbatsov]

CI on master is red on the Emacs 28.2 job, and it has nothing to do with any recent code change. projectile-consult.el ships inside the Projectile package but carried its own Version and Package-Requires headers, so build tooling treats it as a standalone package. A fresh eldev picked up on that and started enforcing Consult's Emacs 29.1 floor across the whole project, so the 28.2 run bails out with "Emacs version 29.1 is required" before a single spec runs. Same double life that caused #2037.

So stop pretending it is a package and drop the two headers. Its runtime needs (Consult 2.0+, hence Emacs 29.1+) are unchanged and still spelled out in the file header, the manual, and the Eldev config, which pulls Consult in only on Emacs 29.1+ and relies on the module byte-compiling cleanly without it elsewhere.

• The commits are consistent with our contribution guidelines
• You've added tests (if possible) to cover your change(s)
• All tests are passing (eldev test)
• The new code is not generating bytecode or M-x checkdoc warnings
• You've updated the changelog (if adding/changing user-visible functionality)
• You've updated the docs (when adding new project types, configuration options, commands, etc)