Media: Remove the variable number (from the big image threshold value) when...

Media: Remove the variable number (from the big image threshold value) when generating file names for scaled images. This makes it easier to "calculate" the full size file name from the name of an intermediate size image.

Props ianmjones, azaozz.
Fixes #48453 for trunk.
Built from https://develop.svn.wordpress.org/trunk@46658


git-svn-id: https://core.svn.wordpress.org/trunk@46458 1a063a9b-81f0-0310-95a4-ce76da25c4cd