isolinux: Drop bootoption "gmt"

We replaced "gmt" with "utc" some time ago, see: 65ee43c.

In the meanwhile we changed the timezone and localtime handling and
dropped support for the bootoption "utc", as it is the default nowadays,
see: grml/grml-autoconfig@e96da70 + 34167e2f.

TZ defaults to UTC and the RTC defaults to be set to UTC.

Related: #296

Author: jkirk

Diff for: templates/boot/isolinux/f4

@@ -9,7 +9,8 @@
  grml blacklist=usbcore                                   disable USB drivers  
  grml debnet        search local partitions for file /etc/network/interfaces,  
                 copy /etc/network then to local system and restart networking  
- grml gmt tz=Europe/Vienna   use GMT-based time and specified timezone for TZ  
+ grml localtime                   configure Hardware Clock / RTC to localtime  
+ grml tz=Europe/Vienna                         set timezone (default: tz=UTC)  
  grml home=/dev/sda1                   use specified device as home directory  
  grml keyboard=de                                                set keyboard  
  grml lang=[at|ch|da|de|es|fr|it|...|us]              set keyboard + language  
@@ -19,5 +20,4 @@
  grml noeject noprompt      do not eject CD after halt / do not prompt for it  
  grml nofstab / nolabel   disable generating of /etc/fstab / use of fs-labels  
 
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 1f