Here's what you should need (I've done this in JIRA v7):
edit the server file:
/opt/atlassian/jira/atlassian-jira/WEB-INF/classes/com/atlassian/jira/web/action/JiraWebActionSupport.properties
Change the following lines as shown (to remove or change the description after the = )
generic.picker.static.desc=
user.picker.ajax.desc=
Note: I am under the belief that as these are system files, these changes may not be persisted in upgrading to new versions. As such, I'm keeping a backup of my changes to re-implement if I notice that they have been overwritten during an install.
Bonus: in that file, these are all of the descriptions I've had removed:
timetracking.field.original.estimate.description=
timetracking.field.remaining.estimate.description=
label.edit.start.typing=
generic.picker.static.desc=
user.picker.ajax.desc=
linkissue.sf.picker.desc=
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