doxygen: remove redundant usages of def

The def command Indicates that a comment block contains documentation
for a #define macro. This is useful if the comment block documents a
macro not adjacent to it, e.g.

```c
/**
 * @def MAX(x,y)
 * @brief Computes the maximum of @a x and @a y.
 */
 #ifdef XXX
 #define MAX(x,y) ...
 #endif
```

However, it is not necessary if the comment is adjacent to the
definition, e.g.

```c
/**
 * @brief Computes the maximum of @a x and @a y.
 */
 #define MAX(x,y) ...
```

This patch removes all unnecessary def entries in-tree.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>