The PSTR is already define as a NOP for all supported platforms. It's
only needed for AVR so here we remove all the uses throughout the
codebase.
This was automated with a simple python script so it also converts
places which spans to multiple lines, removing the matching parentheses.
AVR-specific places were not changed.
Previously this decision was based on the WPNAV_RADIUS parameter which is unexpected (and undocumented) behaviour. Better just to hard-code it to 2m and remove the dependency on this parameter.