Revert "electrum: fixup Exec lines in .desktop"

This reverts commit a879c72552.

It is no longer needed,
as electrum removed the PATH manipulation:
https://github.com/spesmilo/electrum/commit/f15abd7cbd55c82833ad621754d3420895414f75

However, this commit also deviates from a plain revert,
as it adds an additional replacement command to still
substitute the `--testnet` command line, as it was done before.

Note:
Those `--replace` calls are removed here in order to stricten
substitute calls with `--replace-fail` in a follow-up commit.

pkgs/applications/misc/electrum/default.nix

@@ -108,10 +108,8 @@ python3.pkgs.buildPythonApplication rec {
 
   postInstall = lib.optionalString stdenv.hostPlatform.isLinux ''
     substituteInPlace $out/share/applications/electrum.desktop \
-      --replace 'Exec=sh -c "PATH=\"\\$HOME/.local/bin:\\$PATH\"; electrum %u"' \
-                "Exec=$out/bin/electrum %u" \
-      --replace 'Exec=sh -c "PATH=\"\\$HOME/.local/bin:\\$PATH\"; electrum --testnet %u"' \
-                "Exec=$out/bin/electrum --testnet %u"
+      --replace "Exec=electrum %u" "Exec=$out/bin/electrum %u" \
+      --replace "Exec=electrum --testnet %u" "Exec=$out/bin/electrum --testnet %u"
   '';
 
   postFixup = lib.optionalString enableQt ''