remove useless re-declarations of standard functions

Historically, scdoc has used a project-local string.h, which overrode
and masked <string.h> and caused issues.

For example, strstr and strerror were impossible to obtain definitions
for. Instead of fixing the clashing names, the definitions were locally
copied into main.c

In commit 309ff4672b7f5cba9bca1ce8657f767b51f32220 this was fixed
properly (by renaming string.h to str.h) as part of adding printf usage
to the file as well. But the old hacks were never cleaned up.