Remove the --comp option in appimagetool

I'm trying to build an appimage with xz compression due to AppImageLauncher not supporting zstd.
If i run appimagetool-x86_64.AppImage --comp xz .... , it will say "mksquashfs: Compressor "xz" is not supported!"
I believe it's becuase the bundled mksquashfs did not enable any compression methods other than zstd, making --comp meaningless
ref: ci/install-static-mksquashfs.sh
`make -j"$jobs" GZIP_SUPPORT=0 XZ_SUPPORT=0 LZO_SUPPORT=0 LZ4_SUPPORT=0 ZSTD_SUPPORT=1 COMP_DEFAULT=zstd LDFLAGS=-static USE_PREBUILT_MANPAGES=y install`