remove vexp from vendoring process

vendor/github.com/mattn/go-sqlite3/sqlite3_libsqlite3.go

@@ -0,0 +1,13 @@
+// Copyright (C) 2014 Yasuhiro Matsumoto <mattn.jp@gmail.com>.
+//
+// Use of this source code is governed by an MIT-style
+// license that can be found in the LICENSE file.
+// +build libsqlite3
+
+package sqlite3
+
+/*
+#cgo CFLAGS: -DUSE_LIBSQLITE3
+#cgo LDFLAGS: -lsqlite3
+*/
+import "C"