czech: Remove -ům ending in do_case

The Java code removes this ending but it was missing from the Snowball
version.  Looking at the changes resulting from this, it seems a clear
improvement so I've concluded it was an accidental omission.

See snowballstem#151

algorithms/czech.sbl

@@ -79,7 +79,7 @@ backwardmode (
       '{e^}tem' 'at{u*}m'
       '{a'}ch' '{y'}ch' 'ov{e'}' '{y'}mi'
       'ata' 'aty' 'ama' 'ami' 'ovi'
-      'at' '{a'}m' 'os' 'us' '{y'}m' 'mi' 'ou'
+      'at' '{a'}m' 'os' 'us' '{u*}m' '{y'}m' 'mi' 'ou'
       'u' 'y' '{u*}' 'a' 'o' '{a'}' '{e'}' '{y'}'
       (delete)
       'ech' 'ich' '{i'}ch'