Fix: Open scenario editor date query once.

[PeterN]

Motivation / Problem

Click and holding on the scenario editor start date widget causes the query text window to be repeatedly opened until the mouse button is released.

Clearing _left_button_clicked causes the button callback to be fired constantly while the mouse button is held.

Description

This is resolved by removing clearing _left_button_clicked, which turns the button into a regular one-shot.

This is probably a leftover from the year decrement/increment buttons which do need this cleared.

Limitations

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR touches english.txt or translations? Check the guidelines
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
  • ai_changelog.hpp, gs_changelog.hpp need updating.
  • The compatibility wrappers (compat_*.nut) need updating.
• This PR affects the NewGRF API? (label 'needs review: NewGRF')
  • newgrf_debug_data.h may need updating.
  • PR must be added to API tracker