runtime(sbt): do not set b:did_ftplugin before sourcing scala ftplugin(#13657)

The `b:did_ftplugin` guard was set and prevented us from actually sourcing `ftplugin/scala.vim`. Since the latter script also sets the guard properly, we can simply remove the guard here.

Signed-off-by: =?UTF-8?q?Karl=20Yngve=20Lerv=C3=A5g?= <karl.yngve+git@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/ftplugin/sbt.vim b/runtime/ftplugin/sbt.vim
index 309d30e..bf2c7a6 100644
--- a/runtime/ftplugin/sbt.vim
+++ b/runtime/ftplugin/sbt.vim
@@ -9,7 +9,6 @@
   finish
 endif
 
-let b:did_ftplugin = 1
-
 runtime! ftplugin/scala.vim
 
+let b:did_ftplugin = 1