Simplify file transfer section

[ChristinaLK]

Somewhat related to #20, I think we should present one clear option in the file transfer section (https://hpc-carpentry.github.io/hpc-intro/14-transferring-files/) and then refer people to other tools in the "extras" section.

[pdoehle]

+1

[psteinb]

I personally would go with scp for now as it is very close to ssh ;)

[CarrieBrown]

Agree with SCP as the one option offered.

Include a "generic" command format example in the section, such as:
scp <source_host>:<source_file> <target_host>:<target_file>

[CarrieBrown]

Removed sftp in PR #96

Was unsure about the filezilla section, so I left it in for now.

[bkmgit]

Current version of hpc-intro seems clear on this. This issue can be closed.

[tkphd]

@bkmgit please feel free to hit that "Close with comment" button!