[mdns] Drop fallback paths, collapse everything under USE_MDNS_EVENT_DRIVEN_POLLING

mDNS on ESP8266/RP2040 always runs over a network interface (WiFi on ESP8266;
WiFi or W5500/etc. ethernet on RP2040), and every such interface already
publishes an IP state listener in tree. USE_MDNS_EVENT_DRIVEN_POLLING is
therefore always defined in production, so the fallback set_interval() paths
in both platform files and the was_connected_ bookkeeping are dead code.

Also drop the ethernet-specific test — the existing wifi and ethernet+mdns
combos are already covered by the mdns test fixtures paired with their network
component tests.

Trim redundant comments throughout: the header now documents the ~9s
probe/announce window and why update() can stop afterward in a few lines
instead of a full essay; platform files keep only the non-obvious bits (why
RP2040 needs to drive begin()/notifyAPChange() itself, why re-arming on any
listener notification is correct).

tests/components/mdns/common-enabled-ethernet.yaml

@@ -1,23 +0,0 @@
-ethernet:
-  type: W5500
-  clk_pin: 18
-  mosi_pin: 19
-  miso_pin: 16
-  cs_pin: 17
-  interrupt_pin: 21
-  reset_pin: 20
-  manual_ip:
-    static_ip: 192.168.178.56
-    gateway: 192.168.178.1
-    subnet: 255.255.255.0
-  domain: .local
-  mac_address: "02:AA:BB:CC:DD:01"
-
-mdns:
-  disabled: false
-  services:
-    - service: _test_service
-      protocol: _tcp
-      port: 8888
-      txt:
-        static_string: Anything

tests/components/mdns/test-enabled-ethernet.rp2040-ard.yaml

@@ -1 +0,0 @@
-<<: !include common-enabled-ethernet.yaml