Don't mention "serial" in target remote description

Don't mention "serial" in target remote description

PR remote/9177 points out that "info files" mentions "serial" a couple
of times:

    Remote serial target in gdb-specific protocol:
    Debugging a target over a serial line.

However, often the remote target isn't really a serial connection.

It seems to me that this text could be a bit clearer; and furthermore
since "info files" prints the target's long description,
remote_target::files_info doesn't really add much and can simply be
removed.

Regression tested on x86-64 Fedora 34.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=9177