Remove compunit_symtab::m_block_line_section

The previous patch hard-coded SECT_OFF_TEXT into the buildsym code.
After this, it's clear that there is only one caller of
compunit_symtab::set_block_line_section, and it always passes
SECT_OFF_TEXT.  So, remove compunit_symtab::m_block_line_section and
use SECT_OFF_TEXT instead.