prefs: Get rid of line continuations for gui.debounce.timer description

Due to the use of line continuations here, the preference description written into the wireshark preferences file ends up with excessive whitespace.

Before:

% tshark -G defaultprefs | grep -B2 debounce
# How long to wait (in milliseconds) before processing                                   computationally intensive user input.                                   If you type quickly, consider lowering the value for a 'snappier'                                   experience.                                   If you type slowly, consider increasing the value to avoid performance issues.                                   This is currently used to delay searches in View -> Internals -> Supported Protocols                                   and Preferences -> Advanced menu.
# A decimal number
#gui.debounce.timer: 400
%

After:

% ./run/tshark -G defaultprefs | grep -B2 debounce
# How long to wait (in milliseconds) before processing computationally intensive user input. If you type quickly, consider lowering the value for a 'snappier' experience. If you type slowly, consider increasing the value to avoid performance issues.This is currently used to delay searches in View -> Internals -> Supported Protocols and Preferences -> Advanced menu.
# A decimal number
#gui.debounce.timer: 400
%

Fixes: 9e1905f8 ("Preferences: Support configuring debounce timers")