[lldb] [docs] Remove obsolete recommonmark use

[mgorny]

Summary

The recommonmark package is no longer required since all the documents
have been converted to .rst. Remove the related support code from
docs/conf.py.

lldb/docs/conf.py

 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']
 
 # The suffix of source filenames.
 source_suffix = {
     '.rst': 'restructuredtext',
 }
 
-try:
-  import recommonmark
-except ImportError:
-  # manpages do not use any .md sources
-  if not building_man_page:
-    raise
-else:
-  import sphinx
-  if sphinx.version_info >= (3, 0):
-    # This requires 0.5 or later.
-    extensions.append('recommonmark')
-  else:
-    source_parsers = {'.md': 'recommonmark.parser.CommonMarkParser'}
-  source_suffix['.md'] = 'markdown'
-
 # The encoding of source files.
 #source_encoding = 'utf-8-sig'
 
 # The master toctree document.
 master_doc = 'index'
 
 # General information about the project.
 project = u'LLDB'

 # (source start file, name, description, authors, manual section).
 man_pages = [('man/lldb', 'lldb', u'LLDB Documentation', [u'LLVM project'], 1),
              ('man/lldb-server', 'lldb-server', u'LLDB Documentation', [u'LLVM project'], 1),
              ]
 
 # If true, show URL addresses after external links.
 #man_show_urls = False
 
-def process_md(name):
-    file_subpath = os.path.join(command_guide_subpath, name)
-    with open(os.path.join(command_guide_path, name)) as f:
-        title = f.readline().rstrip('\n')
-
-        m = re.match(r'^# (\S+) - (.+)$', title)
-        if m is None:
-            print("error: invalid title in %r "
-                  "(expected '# <name> - <description>')" % file_subpath,
-                  file=sys.stderr)
-        else:
-            man_pages.append((file_subpath.replace('.md',''), m.group(1),
-                              m.group(2), man_page_authors, 1))
-
 # -- Options for Texinfo output ------------------------------------------------
 
 # Grouping the document tree into Texinfo files. List of tuples
 # (source start file, target name, title, author,
 #  dir menu entry, description, category)
 texinfo_documents = [
   ('index', 'LLDB', u'LLDB Documentation',
    u'The LLDB Team', 'LLDB', 'One line description of project.',