Remove the --delete option for --gc

Running `nix-store --gc --delete` will, as of Nix 2.3.3, simply fail
because the --delete option conflicts with the --delete operation.

  $ nix-store --gc --delete
  error: only one operation may be specified
  Try 'nix-store --help' for more information.

Furthermore, it has been broken since at least Nix 0.16 (which was
released sometime in 2010), which means that any scripts which depend
on it should have been broken at least nine years ago. This commit
simply formally removes the option. There should be no actual difference
in behaviour as far as the user is concerned: it errors with the exact
same error message. The manual has been edited to remove any references
to the (now gone) --delete option.

Other information:
* Path for Nix 0.16 used:
  /nix/store/rp3sgmskn0p0pj1ia2qwd5al6f6pinz4-nix-0.16