Remove WebRtcIsac_Highpass_float().

This function is unreferenced and not even declared in a header file.

Split from https://codereview.webrtc.org/1228793004/ .

BUG=none
TEST=none

Review URL: https://codereview.webrtc.org/1296513002

Cr-Commit-Position: refs/heads/master@{#9716}
1 file changed