[doc] install.texi - remove reference to age-old Tcl bug

[Gerald Pfeifer]

Tcl 8.6.1 was released more than half a decade ago, so not much of
a point still documenting this Tcl 8.6 GA regression.

Committed.  

Perhaps I'll backport to the GCC 9 branch as well.

Gerald

2019-08-04  Gerald Pfeifer  <gerald@pfeifer.com>

	* doc/install.texi (Prerequisites): Remove reference to Tcl 8.6
	bug that was fixed in Tcl 8.6.1.

Index: gcc/doc/install.texi
===================================================================
--- gcc/doc/install.texi	(revision 274087)
+++ gcc/doc/install.texi	(working copy)
@@ -442,10 +442,7 @@ Necessary when modifying @command{gperf} input fil
 @itemx Tcl
 
 Necessary to run the GCC testsuite; see the section on testing for
-details.  Tcl 8.6 has a known regression in RE pattern handling that
-make parts of the testsuite fail.  See
-@uref{http://core.tcl.tk/tcl/tktview/267b7e2334ee2e9de34c4b00d6e72e2f1997085f}
-for more information.  This bug has been fixed in 8.6.1.
+details.
 
 @item autogen version 5.5.4 (or later) and
 @itemx guile version 1.4.1 (or later)