pihole: remove ca cert

apparently this is not needed anymore an even makes TLS requests fail
with an internal error:

2024-02-18 08:24:59: mod_openssl.c.1095) SSL: building cert chain for TLS server name pihole.home.arpa: error:00000000:lib(0):func(0):reason(0)
2024-02-18 08:24:59: mod_openssl.c.3095) SSL: 1 error:1417A179:SSL routines:tls_post_process_client_hello:cert cb error

files/opt/pihole/lighttpd.conf

@@ -8,7 +8,6 @@ $HTTP["host"] == "pihole.home.arpa" {
   $SERVER["socket"] == ":443" {
     ssl.engine = "enable"
     ssl.pemfile = "/run/lighttpd/combined.pem"
-    ssl.ca-file =  "/run/lighttpd/ca.pem"
     ssl.honor-cipher-order = "enable"
     ssl.cipher-list = "EECDH+AESGCM:EDH+AESGCM:AES256+EECDH:AES256+EDH"
   }