Remove hard coded charset.

Remove hard coded charset.

In thinking about the proposal here
https://github.com/civicrm/civicrm-core/pull/13633/files#diff-c5e39924ad99ac88b48763a0d8fa246eR30
to switch to utf8mb4 I found that none of the places on the web that suggest functions like ours
specifiy charset - eg. https://stackoverflow.com/questions/287105/mysql-strip-non-numeric-characters-to-compare

When I look at the code it is iterating through the characters to find integers - which should
be the same regardless of charset. So, I think we can simply remove it.
: