Fix execute-extended-command help text for remapped commands

* lisp/simple.el (execute-extended-command): Don't provide
misleading help text for remapped commands (bug#23543).
10 jobs for master in 324 minutes and 6 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Build Images
passed #46941
build-image-inotify

03:10:35

 
  Normal
passed #46942
test-all-inotify

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  Platform Images
passed #46943
build-image-filenotify-gio

03:16:31

failed #46944
build-image-gnustep

04:00:03

 
  Platforms
passed #46945
test-filenotify-gio

00:15:37

skipped #46946
test-gnustep
 
  Native Comp Images
passed #46947
build-native-comp-speed0

02:02:06

passed #46948
build-native-comp-speed1

05:24:03

passed #46949
build-native-comp-speed2

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  Native Comp
passed #46950
test-native-comp-speed0

01:05:32

 
Name Stage Failure
failed
build-image-gnustep Platform Images The script exceeded the maximum execution time set for the job
be1afa587864: Retrying in 7 seconds
be1afa587864: Retrying in 6 seconds
be1afa587864: Retrying in 5 seconds
be1afa587864: Retrying in 4 seconds
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Pulling docker image gitlab/gitlab-runner-helper:x86_64-d0b76032 ...
ERROR: Job failed: execution took longer than 4h0m0s seconds