how to remove untracked files in Git?

To remove untracked files / directories do:

git clean -fdx

-f - force

-d - directories too

-x - remove ignored files too ( don't use this if you don't want to remove ignored files)

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Use with Caution!
These commands can permanently delete arbitrary files, that you havn't thought of at first. Please double check and read all the comments below this answer and the --help section, etc., so to know all details to fine-tune your commands and surely get the expected result.

User interactive approach:

git clean -i -fd

Remove .classpath [y/N]? N
Remove .gitignore [y/N]? N
Remove .project [y/N]? N
Remove .settings/ [y/N]? N
Remove src/com/amazon/arsdumpgenerator/inspector/ [y/N]? y
Remove src/com/amazon/arsdumpgenerator/manifest/ [y/N]? y
Remove src/com/amazon/arsdumpgenerator/s3/ [y/N]? y
Remove tst/com/amazon/arsdumpgenerator/manifest/ [y/N]? y
Remove tst/com/amazon/arsdumpgenerator/s3/ [y/N]? y

-i for interactive
-f for force
-d for directory
-x for ignored files(add if required)

Note: Add -n or --dry-run to just check what it will do.