Make export.fish exit with SUCCESS exit code

The export.fish script exits with an exit code of 4. Thus, any shell checks that make sure the source command exits successfully always failed. This was due to the last line trying to erase the __main function. In fish, you can't erase a function using the `set` command, you can only erase variables. By removing that line the script now exits with an exit code of 0 instead of 4.

Erase __main function at the end of export.fish

Closes https://github.com/espressif/esp-idf/pull/10828

export.fish

@@ -107,5 +107,4 @@ else
     eval (env _IDF.PY_COMPLETE=fish_source idf.py)
 end
 
-
-set -e __main
+functions -e __main