Retire `finddate.sh` script usage

[KateFriedman-NOAA]

Remove reference to and usage of finddate.sh. Companion issue to NOAA-EMC/global-workflow#2279.

[KateFriedman-NOAA]

Instances of finddate.sh to remove, including the actual script:

ush/finddate.sh:# finddate.sh
ush/finddate.sh:#     list=`sh /nwprod/util/scripts/finddate.sh 19990929 s+10`
ush/finddate.sh:#     list=`sh /nwprod/util/scripts/finddate.sh 19990929 d+10`
ush/finddate.sh:#     list=`sh /nwprod/util/scripts/finddate.sh 19990929 s-10`
ush/finddate.sh:#     list=`sh /nwprod/util/scripts/finddate.sh 19990929 d-10`
ush/finddate.sh:# Copy of finddate.sh
ush/finddate.sh:function finddate() {
ush/finddate.sh:# The old finddate.sh didn't provide functions, so call the main function here
ush/finddate.sh:finddate "$1" "$2"
ush/CMakeLists.txt:  finddate.sh

Also look at:

ush/test_finddate.sh:tap_start_subtest "finddate matches old output" 4
ush/test_finddate.sh:declare +i actual="$(finddate "${start_date}" s+10)"
ush/test_finddate.sh:actual="$(finddate "${start_date}" d+10)"
ush/test_finddate.sh:actual="$(finddate "${start_date}" s-10)"
ush/test_finddate.sh:actual="$(finddate "${start_date}" d-10)"
ush/test_finddate.sh:tap_end_subtest "finddate matches old output"
ush/test_finddate.sh:tap_start_subtest "finddate extreme examples" 3
ush/test_finddate.sh:declare -i actual="10#$(finddate "${start_date}" d+366)"
ush/test_finddate.sh:actual="10#$(finddate "${start_date}" d+3653)"
ush/test_finddate.sh:actual="10#$(finddate "${start_date}" d+7305)"
ush/test_finddate.sh:tap_end_subtest "finddate extreme examples"