Vestigial reference to RAFT_LIBS in dqlite.pc.in

[gibmat]

With the release of dqlite 1.18.0, raft is now an entirely internal implementation detail. However, there's still a reference to RAFT_LIBS in dqlite.pc.in:

diff --git a/dqlite.pc.in b/dqlite.pc.in
index 2934626..cae8217 100644
--- a/dqlite.pc.in
+++ b/dqlite.pc.in
@@ -7,5 +7,5 @@ Name: dqlite
 Description: Distributed SQLite engine
 Version:  @PACKAGE_VERSION@
 Libs: -L${libdir} -ldqlite
-Libs.private: @SQLITE_LIBS@ @UV_LIBS@ @RAFT_LIBS@
+Libs.private: @SQLITE_LIBS@ @UV_LIBS@
 Cflags: -I${includedir}