Cleans up src folder after download and build

[stigi]

Removing src after building, so that the install script does not install it as well.

The install script will otherwise copy the entire contents of $ASDF_DOWNLOAD_PATH into the installation directory. This causes a lot of confirmation steps on my maching:

$ asdf install sourcedocs latest
[...]
Build complete! (25.27s)
sourcedocs 2.0.2 installation was successful!
override r--r--r-- ullrich/staff for /Users/ullrich/.asdf/downloads/sourcedocs/2.0.2/src/SourceDocs-2.0.2/.build/checkouts/swift-argument-parser/CODE_OF_CONDUCT.md? y
override r--r--r-- ullrich/staff for /Users/ullrich/.asdf/downloads/sourcedocs/2.0.2/src/SourceDocs-2.0.2/.build/checkouts/swift-argument-parser/Tools/changelog-authors/Util.swift? y
override r--r--r-- ullrich/staff for /Users/ullrich/.asdf/downloads/sourcedocs/2.0.2/src/SourceDocs-2.0.2/.build/checkouts/swift-argument-parser/Tools/changelog-authors/Models.swift? y
[...a thousand more...]