Site Health: Remove `parse_ini_size()`, use the existing...

Site Health: Remove `parse_ini_size()`, use the existing `wp_convert_hr_to_bytes()` function instead.

Follow-up to [48535].

See #50038.
Built from https://develop.svn.wordpress.org/trunk@48538


git-svn-id: https://core.svn.wordpress.org/trunk@48300 1a063a9b-81f0-0310-95a4-ce76da25c4cd