Clean up legacy bytecode loading

Clean up legacy bytecode loading

Complete some unfinished work from the elimination of lazy bytecode
loading in 9bcc9690a8: remove code which became unreachable by that
reform.  This simplifies some of the lower-level parts of the reader
which helps performance somewhat.

* src/doc.c (read_bytecode_pointer, read_bytecode_char)
(read_doc_string): Remove.
(get_doc_string): Remove last argument, all callers adapted.  Simplify.
* src/lread.c (readbyte_for_lambda, readbyte_from_string)
(readbyte_for_lambda, readbyte_from_string): Remove.
(readchar, unreadchar): Remove unused code paths for Qlambda and cons
arguments.
(bytecode_from_rev_list): Remove last use of a quirky code path,
decoding force-loaded lazy bytecode explicitly.

(cherry picked from commit 236034a24d0e93e1796224e2ff4a14819fcfd559)