Remove fish.desktop file as it was only needed for AppImages which were decided against.

cmake/Install.cmake

@@ -165,7 +165,6 @@ if(GETTEXT_FOUND)
 endif()
 
 if (NOT APPLE)
-    install(FILES fish.desktop DESTINATION ${rel_datadir}/applications)
     install(FILES ${SPHINX_SRC_DIR}/python_docs_theme/static/fish.png DESTINATION ${rel_datadir}/pixmaps)
 endif()
 

fish.desktop

@@ -1,12 +0,0 @@
-[Desktop Entry]
-Version=1.0
-Type=Application
-Terminal=true
-# This file is for use in appimages, which rewrite the path.
-# Other uses would have to replace these with the correct path.
-Exec=fish
-Name=fish
-Comment=The user-friendly command line shell
-Icon=fish
-# "Shell" would be the proper category, but it's reserved so we can't use it.
-Categories=ConsoleOnly;System;