gdb: remove buildsym_compunit::record_block_range

After the last commit I realised that the function
buildsym_compunit::record_block_range is not really needed any more.
This function is basically checking for non-contiguous blocks, and
setting m_pending_addrmap_interesting when a non-contiguous block is
seen.

We can move this check into buildsym_compunit::make_blockvector which
is the only place m_pending_addrmap_interesting is used, we already
loop over all blocks in this function.

This allows us to remove record_block_range and
m_pending_addrmap_interesting from buildsym_compunit.

There is one weird thing in this patch, in dwarf2/read.c, the function
dwarf2_record_single_block_range is now empty, but I've retained this
function.  A later patch in this series adds some code back into this
function, so I don't want to delete it here.