Revert "bootstrap.conf: require bison 3.6 or later"

This reverts commit b6de8e01dec78ac0a43bccae49d1ecda64ad9b98.  The
condition was too rigid on oss-fuzz.  This was introduced for the
default string change ("$eof" -> "end of file"), but ideally we should
have a way to produce the same error message for either version of
bison.

Signed-off-by: Daiki Ueno <ueno@gnu.org>
diff --git a/bootstrap.conf b/bootstrap.conf
index c8a85b8..45e9689 100644
--- a/bootstrap.conf
+++ b/bootstrap.conf
@@ -42,7 +42,7 @@
 autoconf   2.62
 automake   1.11.1
 git        1.4.4
-bison      3.6
+bison      -
 "
 
 GTKDOCIZE=$(which gtkdocize 2>/dev/null)