[incsrch] remove gpl.txt

This is still GPL as indicated in every source header, but there is
no need to duplicate license in every subdirectory. It was there
because a plugin for Windows FAR is a separate download, but in the
source tree of far2l this is excessive. One GPL license per far2l
source tree in enough.

Moreover, I hereby authorize far2l project maintainers to swicth a
license as they deem suitable to any free license for the plugin as
long as it is the same as the far2l itself. This authorization extends
to the changes to the sources headers if deemed necessary.

Author: rampitec