all: simplify gofmt script (stellar#1777)

Remove the ls xargs dance from the gofmt and just get gofmt to run it's checks over everything of what's different.

I was looking at copying this script to another repository and noticed that the dance with excluding certain folders and piping to xargs to run in parallel is really not necessary.

I'm not making this change to improve the performance of this, because I don't think it's meaningful right here, but just mentioning this for completeness that when I run the script as it is now sharing across 4 processes it was actually marginally slower than just running gofmt once like this change does. So this change isn't introducing a slowdown in CI runtime.

gofmt.sh

@@ -2,9 +2,7 @@
 set -e
 
 printf "Running gofmt checks...\n"
-OUTPUT=$(ls -d */ \
-  | egrep -v '^vendor|^docs' \
-  | xargs -I {} -P 4 gofmt -d {})
+OUTPUT=$(gofmt -d .)
 
 if [[ $OUTPUT ]]; then
   printf "gofmt found unformatted files:\n\n"