Bluetooth: ISO: Remove CONFIG_BT_ISO_TX_MTU

[Thalley]

Is your enhancement proposal related to a problem? Please describe.
CONFIG_BT_ISO_TX_MTU today is not used by the host.

It has 3 uses

1. Many samples and tests are using this value to define their TX buffer pools
2. hci_raw.c uses it to define the size of the hci_iso_pool.
3. The controller optionally uses it to determine the HCI ISO TX buffer size, but CONFIG_BT_CTLR_ISO_TX_BUFFER_SIZE can be directly for that too

Describe the solution you'd like
Remove the Kconfig as the host doesn't it and the applications shouldn't. For hci_raw and the HCI ISO tx buffer sizes we can/should use CONFIG_BT_CTLR_ISO_TX_BUFFER_SIZE.

Describe alternatives you've considered
N/A

Additional context
N/A

[kruithofa]

Before doing this we need to add a babblesim test with HCI packets larger than the HCI MTU size, basically testing fragmented HCI packets

[Thalley]

PR adding the fragmented test: #81871