lib-sieve: Drop system error handler.

src/lib-sieve/sieve-common.h

src/lib-sieve/sieve-error.c

src/lib-sieve/sieve-error.h

src/plugins/imap-filter-sieve/imap-filter-sieve.c

src/plugins/imapsieve/imap-sieve.c

src/plugins/lda-sieve/lda-sieve-plugin.c

src/sieve-tools/sieve-filter.c

src/sieve-tools/sieve-test.c

src/testsuite/testsuite-log.c