logging: backend_fs: disable ANSI color for filesystem logging

Before the fix for #77150 was merged, with LOG_BACKEND_SHOW_COLOR=n and
CONFIG_SHELL_VT100_COLORS=y the shell would still display colors by
enabling the flag in the shell_log_backend.c.

Now, LOG_BACKEND_SHOW_COLOR has to be enabled to allow shell logging
color, but that writes ANSI escape sequences to the log files.

Explicitly disable ANSI color output for the filesystem logging backend.

Signed-off-by: Eric Holmberg <eric.holmberg@northriversystems.co.nz>