Revert "chan_sip: Fix lastrtprx always updated"

This reverts commit 93332cb1d0.

Unfortunately, the aforementioned commit caused a regression (incoming calls
would eventually disconnect). Thus it is being removed.

ASTERISK-26523 #close
ASTERISK-25270

Change-Id: Ibf5586adc303073a8eac667a4cbfdb6be184a64d

channels/chan_sip.c

@@ -8633,9 +8633,7 @@ static struct ast_frame *sip_read(struct ast_channel *ast)
 
 	sip_pvt_lock(p);
 	fr = sip_rtp_read(ast, p, &faxdetected);
-	if (fr && fr->frametype != AST_FRAME_NULL) {
-		p->lastrtprx = time(NULL);
-	}
+	p->lastrtprx = time(NULL);
 
 	/* If we detect a CNG tone and fax detection is enabled then send us off to the fax extension */
 	if (faxdetected && ast_test_flag(&p->flags[1], SIP_PAGE2_FAX_DETECT_CNG)) {