ICU-22900 Remove deprecated SimpleNumber truncateStart #2860

Job	Run time
clang-valgrind-test	10m 13s
clang-valgrind-intltest (utility)	7m 17s
clang-valgrind-intltest (normalize)	6m 11s
clang-valgrind-intltest (collator)	5m 37s
clang-valgrind-intltest (regex)	5m 35s
clang-valgrind-intltest (format)	16m 19s
clang-valgrind-intltest (translit)	6m 5s
clang-valgrind-intltest (rbbi)	7m 40s
clang-valgrind-intltest (rbnf)	6m 20s
clang-valgrind-intltest (rbnfrt)	6m 15s
clang-valgrind-intltest (icuserv)	5m 18s
clang-valgrind-intltest (idna)	5m 7s
clang-valgrind-intltest (convert)	7m 10s
clang-valgrind-intltest (rbnfp)	5m 23s
clang-valgrind-intltest (csdet)	5m 30s
clang-valgrind-intltest (spoof)	5m 2s
clang-valgrind-intltest (bidi)	5m 36s
	1h 56m 38s

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