s3: winbind: Remove fstring from wb_acct_info struct

The group enumeration backend functions try to allocate an array of
wb_acct_info structs with a number of elements equal to the number of
groups. In domains with a large number of groups this allocation may
fail due to the size of the chunk.

Found while trying to enumerate the groups in a domain with more than
700k groups.

Signed-off-by: Samuel Cabrero <scabrero@suse.de>
17 jobs for winbind_enum_grp_nomem in 119 minutes and 5 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed build_ctdb #114839963
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passed build_others #114839971
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passed build_samba_ad_dc_2 #114839955
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passed build_samba_ad_dc_2_py3 #114839956
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passed build_samba_buildpy3_only #114839972
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passed build_samba_ctdb #114839969
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passed build_samba_libs #114839957
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passed build_samba_libs_py3 #114839958
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passed build_samba_none_env #114839945
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passed build_samba_nopython #114839946
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passed build_samba_o3 #114839954
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passed build_samba_purepy3-none_env #114916404
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passed build_samba_static #114839961
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passed build_samba_systemkrb5 #114839948
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passed build_samba_xc #114839951
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failed build_samba_purepy3-none_env #114861787
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failed build_samba_purepy3-none_env #114839973
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