install.sh:fetch: don't use rm -rf with variable arguments

[SwuduSusuwu]

Purposes

• Prevents massive filesystem destruction if root directory is passed.
• The argument is the basename of the Uniform Resource Identifier passed to curl, which curl should not store as RO (thus no -f flag needed to remove).
• The argument is a single file (thus no -r flag needed to remove).

[SwuduSusuwu]

Can clone and make_install use just -r (can have the f flag removed?):

• install.sh:clone
• install.sh:make_install