maint: Remove functions deprecated in 3.6 for 4.0 release. * NEWS: Add notice about removal of deprecated functions. * etc/NEWS.3: Correct deprecation list from 3.6 which incorrectly named is_duplicate_entry. * scripts/deprecated/module.mk: Remove deprecated scripts from build system. * scripts/deprecated/__error_text__.m, scripts/deprecated/cor.m, scripts/deprecated/corrcoef.m, scripts/deprecated/cut.m, scripts/deprecated/error_text.m, scripts/deprecated/polyderiv.m, scripts/deprecated/shell_cmd.m, scripts/deprecated/studentize.m, scripts/deprecated/sylvester_matrix.m: Remove files from source control.

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