You can quickly delete all of these styles with a simple macro:
REM Remove the cell styles that begin with prefixStyleName
Sub removeStyles()
Dim oStyleFamilies As Variant
Dim oStyleFamilie As Variant
Dim aElementNames As Variant
Dim i%, count%
Const prefixStyleName = "Excel_CondFormat_"
	oStyleFamilies = ThisComponent.getStyleFamilies()
	oStyleFamilie = oStyleFamilies.getByName("CellStyles")
	aElementNames = oStyleFamilie.getElementNames()
	count = 0
	For i = LBound(aElementNames) To UBound(aElementNames)
		If Left(aElementNames(i), Len(prefixStyleName)) = prefixStyleName Then
			oStyleFamilie.removeByName(aElementNames(i))
			count = count + 1
		EndIf
	Next i
	MsgBox ("It was removed " + count + " style(s) with the names '" + prefixStyleName + "...'", 64, "Completed successfully")
End Sub
However, you must remember that you lose a set of conditional formats in the document. The conditions for the cells remain in the same form as in the original xls-file, but the design of the cells which correspond to these conditions will be “undefined”