libfm: fix duplicate inclusion of libfm-extra

[wamserma]

Motivation for this change

libfm-extra is pulled in implicitly when libfm is built. Remove duplicate files.

collision between `/nix/store/cas1jifhs1k2hdrs3mczpfs67x8f9wjf-libfm-extra-1.3.1/lib/libfm-extra.so' and `/nix/store/k88h4xr33adf9kw12pfjgp6lwnr4wxyr-libfm-1.3.1/lib/libfm-extra.so'
collision between `/nix/store/cas1jifhs1k2hdrs3mczpfs67x8f9wjf-libfm-extra-1.3.1/lib/libfm-extra.la' and `/nix/store/k88h4xr33adf9kw12pfjgp6lwnr4wxyr-libfm-1.3.1/lib/libfm-extra.la'
collision between `/nix/store/cas1jifhs1k2hdrs3mczpfs67x8f9wjf-libfm-extra-1.3.1/lib/libfm-extra.so.4.1.2' and `/nix/store/k88h4xr33adf9kw12pfjgp6lwnr4wxyr-libfm-1.3.1/lib/libfm-extra.so.4.1.2'
collision between `/nix/store/cas1jifhs1k2hdrs3mczpfs67x8f9wjf-libfm-extra-1.3.1/lib/libfm-extra.so.4' and `/nix/store/k88h4xr33adf9kw12pfjgp6lwnr4wxyr-libfm-1.3.1/lib/libfm-extra.so.4'
collision between `/nix/store/cas1jifhs1k2hdrs3mczpfs67x8f9wjf-libfm-extra-1.3.1/lib/pkgconfig/libfm-extra.pc' and `/nix/store/k88h4xr33adf9kw12pfjgp6lwnr4wxyr-libfm-1.3.1/lib/pkgconfig/libfm-extra.pc'

nix why-depends nixpkgs.pcmanfm nixpkgs.libfm-extra
/nix/store/ccs0xsvqhiishcd0acs2plikci1fcz2g-pcmanfm-1.3.1
╚═══bin/.pcmanfm-wrapped: …p-harfbuzz-2.6.4/lib:/nix/store/pm5k7dx68jwpbv0cv4wzk3zcika0x2pj-libfm-1.3.1/lib:/nix/store/9hy6…
    => /nix/store/pm5k7dx68jwpbv0cv4wzk3zcika0x2pj-libfm-1.3.1
    ╚═══bin/libfm-pref-apps: …p-harfbuzz-2.6.4/lib:/nix/store/8ls43gqsslh8ky0qr524cnwy7wwh9zbi-menu-cache-1.1.0/lib:/nix/store…
        => /nix/store/8ls43gqsslh8ky0qr524cnwy7wwh9zbi-menu-cache-1.1.0
        ╚═══libexec/menu-cache/menu-cache-gen: …LIBC_2.7.GLIBC_2.3.4./nix/store/cas1jifhs1k2hdrs3mczpfs67x8f9wjf-libfm-extra-1.3.1/lib:/nix/stor…
            => /nix/store/cas1jifhs1k2hdrs3mczpfs67x8f9wjf-libfm-extra-1.3.1

Things done

• compiled and tested pcmanfm
• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.