Auto merge of #38144 - clarcharr:redundant, r=alexcrichton
Auto merge of #38144 - clarcharr:redundant, r=alexcrichton

Remove redundant assertion near is_char_boundary

Follow-up from #38056. `is_char_boundary` already checks for `idx <= len`, so, an extra assertion is redundant.

graydon/rust,GBGamer/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,GBGamer/rust,aidancully/rust,graydon/rust