ICU-22900 Remove deprecated SimpleNumber truncateStart #2853

Job	Run time
clang-valgrind-test	13s
clang-valgrind-intltest (utility)	15s
clang-valgrind-intltest (normalize)	1s
clang-valgrind-intltest (collator)	14s
clang-valgrind-intltest (regex)	13s
clang-valgrind-intltest (format)	12s
clang-valgrind-intltest (translit)	14s
clang-valgrind-intltest (rbbi)	10s
clang-valgrind-intltest (rbnf)	14s
clang-valgrind-intltest (rbnfrt)	10s
clang-valgrind-intltest (icuserv)	2s
clang-valgrind-intltest (idna)	13s
clang-valgrind-intltest (convert)	3s
clang-valgrind-intltest (rbnfp)	15s
clang-valgrind-intltest (csdet)	15s
clang-valgrind-intltest (spoof)	2s
clang-valgrind-intltest (bidi)	2s
	2m 48s

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