libcaja-private: fix always-false expressions

CppCheck detected a few 'always false' expressions in caja.

First two definitely appeareed due to copy-paste (mime-types
comparison). It would be nice to move x_content_type's and corresponding
greetings to some table (array of structs), wouldn't it?

The last one looks like copy-paste problem too. There were two identical
blocks of code in move_file_prepare() checking conflict type. BTW, there
is exactly the same code in the same file inside copy_move_file().
According to DRY, it would be nice to move this code to its own
function.