vinyl: remove unused error injection handling in cache

ERRINJ_VY_READ_PAGE and ERRINJ_VY_READ_PAGE_TIMEOUT injections are
used by vinyl/errinj test to check that a page read error is handled
properly. The cache checks if these injections are enabled and bails
out if so. Since commit a31c2c10 ("vinyl: force read view in
iterator in autocommit mode"), this is not necessary, because cache
is not used unless SELECT is called from a transaction, and the above
mentioned test doesn't use transactions. So let's remove the checks.
If we ever enable cache for all SELECTs, we can disable cache in the
test with box.cfg.vinyl_cache instead of using error injections.