$ myscript "My name's Santiago. What's yours?" "atu"
My nme's Snigo. Wh's yors?

$ cat myscript
ans=$1
for (( i=0; i<${#2}; i++ )); do
    ans=${ans//${2:$i:1}/}
done
echo "$ans"

~$ cat myscript
echo "$1" | tr -d "${2//\\/\\\\}"
~$ myscript 'weird string !"#$%&'\''()*+,-./:;<=>?@[\]^_`{|}~' ' !"#$%&'\''()*+,-./:;<=>?@[\]^_`{|}~'
            <---------------------- $1 ----------------------> <---------------- $2 ---------------->
weirdstring
~$

echo "My name's Santiago. What's yours?" | tr -d "atu"