ICU-22900 Remove deprecated SimpleNumber truncateStart #2855

Job	Run time
clang-valgrind-test	9m 55s
clang-valgrind-intltest (utility)	6m 49s
clang-valgrind-intltest (normalize)	6m 17s
clang-valgrind-intltest (collator)	5m 13s
clang-valgrind-intltest (regex)	5m 37s
clang-valgrind-intltest (format)	16m 4s
clang-valgrind-intltest (translit)	5m 54s
clang-valgrind-intltest (rbbi)	7m 34s
clang-valgrind-intltest (rbnf)	6m 0s
clang-valgrind-intltest (rbnfrt)	6m 32s
clang-valgrind-intltest (icuserv)	5m 12s
clang-valgrind-intltest (idna)	5m 40s
clang-valgrind-intltest (convert)	7m 3s
clang-valgrind-intltest (rbnfp)	5m 0s
clang-valgrind-intltest (csdet)	5m 38s
clang-valgrind-intltest (spoof)	5m 2s
clang-valgrind-intltest (bidi)	7m 34s
	1h 57m 4s

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