Remove some useless casts from and to void pointers

Remove some useless casts from and to void pointers

Remove casts from/to void*.  They are unneeded in C, can hide problems
in the future, and are far too C++ish.  Furthermore, they were
inconsistent with the rest of the code and even with regards to
themselves (at least in terms of whether or not to have space after the
cast).

In this case, we temporarily lose const specifiers in libdivecomputer.c
due to the unneeded cast, so it seems better to avoid the cast at all,
so you get warned about a const->non-const cast if you ever change it to
do something like this.

The casts in gtk-gui.c are just useless semantically, although they
might be useful as a hint to the reader that the void pointers are char
arrays.

Signed-off-by: Julian Andres Klode <jak@jak-linux.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>