Fix use of undefined const NS_TIMEDTEXT

NS_TIMEDTEXT is only set if $wgEnableLocalTimedText is true, in which case it
is defined to have the same value as $wgTimedTextNS. If $wgEnableLocalTimedText
is not true, the const is not defined, and $wgTimedTextNS is reset to false.
Comparing a namespace ID to $wgTimedTextNS is therefore the proper way to test
for NS_TIMEDTEXT.

Change-Id: I2d893176f06555fd18475098bcff2d556d416d68
Merge "Fix use of undefined const NS_TIMEDTEXT"