index.sh: Remove `linux64-static` override

[JoeKar]

Description

The linux64-static override can be dropped since micro's binary files will be build fully static in the future (see: zyedidia/micro#3466). Due to this there is no need for a musl libc check and the override of linux64 with linux64-static.

Unfortunately this will take effect earliest with the upcoming release.

How Has This Been Tested?

$ ldd micro
  das Programm ist nicht dynamisch gelinkt

Checklist:

• [x] I have commented my code, particularly in hard-to-understand areas
• [x] If I added new user-facing functionality, I have made corresponding changes to the README documenting the changes
• [x] If this is a code change, I have updated the test configuration and/or test scripts to cover the changes I've made.