net-im/bitlbee: remove obsolete xmpp? ( !nss ) requirement

This requirement was basically introduced with 1.2 release in commit [1]
in order to solve https://bugs.gentoo.org/213958#c8 issue (the xmpp use
flag was called jabber back then). The requirement was later transformed
to REQUIRED_USE in commit [2]. However, this requirement seems no longer
be valid.

The nss use description in metadata was also bogus, it is changed to
more generic form.

[1] https://gitweb.gentoo.org/repo/gentoo/historical.git/commit/?id=30a0fafed3bd420fdb03e6e0a0da3d5f0a86b62e
[2] https://gitweb.gentoo.org/repo/gentoo/historical.git/commit/?id=65aa90800da80b1fbffc36c7b15c3503c37d7fd0

Bug: https://bugs.gentoo.org/213958
Signed-off-by: Petr Vaněk <arkamar@atlas.cz>
Signed-off-by: Sam James <sam@gentoo.org>

net-im/bitlbee/bitlbee-3.6-r2.ebuild

@@ -29,7 +29,6 @@ REQUIRED_USE="
 	|| ( purple xmpp )
 	purple? ( plugins )
 	test? ( xmpp )
-	xmpp? ( !nss )
 "
 
 COMMON_DEPEND="

net-im/bitlbee/bitlbee-3.6-r3.ebuild

@@ -29,7 +29,6 @@ REQUIRED_USE="
 	|| ( purple xmpp )
 	purple? ( plugins )
 	test? ( xmpp )
-	xmpp? ( !nss )
 "
 
 COMMON_DEPEND="

net-im/bitlbee/bitlbee-9999.ebuild

@@ -29,7 +29,6 @@ REQUIRED_USE="
 	|| ( purple xmpp )
 	purple? ( plugins )
 	test? ( xmpp )
-	xmpp? ( !nss )
 "
 
 COMMON_DEPEND="

net-im/bitlbee/metadata.xml

@@ -11,7 +11,7 @@
 	</maintainer>
 	<use>
 		<flag name="libevent">Use libevent for event handling</flag>
-		<flag name="nss">Use NSS for SSL support in Jabber</flag>
+		<flag name="nss">Prefer <pkg>dev-libs/nss</pkg> as SSL/TLS provider</flag>
 		<flag name="otr">Enable support for encrypted conversations</flag>
 		<flag name="plugins">Enable support for plugins</flag>
 		<flag name="purple">Use libpurple instead of the built-in IM protocol support</flag>