Eliminate DQUOT_START/SQUOT_START tokens

Quoted strings are naturally terminated by end marker, potentially
preceded by DQUOT_STRING/SQUOT_STRING. This renders the corresponding
start tokens really just a academic completeness, costing us memory
while not bringing anything to the table.

Skip generation of these tokens, reducing memory usage by up to 4.5%.

JIRA: YANGTOOLS-1089
Change-Id: I0b7ce9bf292b0dd8475d63869ddfd8d2b86a387c
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit 92a6c34fae26e54f9cf962228374805198b65059)