remove env:FYSETC_F6

[ellensp]

Description

While looking at an issue for a user using a BOARD_FYSETC_F6_14 that had an issue where all but one TMC2209 stepper driver reported errors.

M122
12:38:29.768 > Testing X connection... Error: All LOW
12:38:29.787 > Testing Y connection... OK
12:38:29.796 > Testing Z connection... Error: All LOW
12:38:29.817 > Testing E connection... Error: All LOW

This was traced to the "board = fysetc_f6_13" being incomplete. This is a FYSETC provided board definition via platformio

Short version pins 72,75,76 and 77 were not setup correctly for change interrupt.
But 73 was setup correctly, and this is what Y UART uses

See for FYSETC/FYSETC-F6#3 (comment) details
I then found that other people have been triyng to get this sorted since 2020 platformio/platform-atmelavr#180 without success.

I then did a deep dive into what the board fysetc_f6_13 actually changed vs our variant MARLIN_MEGA_EXTENDED and found that env:mega2560ext already does everything required.

So I have removed all references to env:FYSETC_F6 and updated both boards to use env:mega2560ext
This fixes the issue for Marlin users now without having to wait for others to fix things.

Requirements

BOARD_FYSETC_F6_13 or BOARD_FYSETC_F6_14

Benefits

Uses a build environment that has all required features.
TMC2208/TMC2209 stepper drivers work as expected.

Related Issues

• Add FYSETC F6 V1.4 platformio/platform-atmelavr#180
• board fysetc_f6_13 pins_arduino.h out of date platformio/platform-atmelavr#346
• FYSETC F6 V1.3 TMC CONNECTION ERROR FYSETC/FYSETC-F6#3