Modifications to CODEOWNERS file (ufs-community#757)

* Add @gspetro-NOAA, @natalie-perlin, and @EdwardSnyder-NOAA to CODEOWNERS so they are notified of all PRs and can review them.

* Remove duplicates in CODEOWNERS; remove users who will no longer be working with the repo.

.github/CODEOWNERS

@@ -3,7 +3,7 @@
 
 # These owners will be the default owners for everything in the repo.
 #*       @defunkt
-*       @mkavulich @gsketefian @JeffBeck-NOAA @RatkoVasic-NOAA @BenjaminBlake-NOAA @ywangwof @chan-hoo @BenjaminBlake-NOAA @RatkoVasic-NOAA @panll @christinaholtNOAA @christopherwharrop-noaa @danielabdi-noaa @mark-a-potts @jkbk2004 @willmayfield @jwolff-ncar @dmwright526
+*       @mkavulich @gsketefian @JeffBeck-NOAA @RatkoVasic-NOAA @BenjaminBlake-NOAA @ywangwof @chan-hoo @panll @christinaholtNOAA @christopherwharrop-noaa @danielabdi-noaa @mark-a-potts @jkbk2004 @willmayfield @dmwright526 @gspetro-NOAA @natalie-perlin @EdwardSnyder-NOAA
 
 # Order is important. The last matching pattern has the most precedence.
 # So if a pull request only touches javascript files, only these owners