Delete previous MIP starts if they exist

[odow]

This line adds a new MIP start

CPLEX.jl/src/MOI/MOI_wrapper.jl

Lines 2458 to 2469 in 71e64ea

	if length(varindices) > 0
	ret = CPXaddmipstarts(
	model.env,
	model.lp,
	1,
	length(varindices),
	Ref{Cint}(0),
	varindices,
	values,
	Ref{Cint}(CPX_MIPSTART_AUTO),
	C_NULL,
	)

Bit it adds a mip-start, leaving the previous one in-place.

Looks like we need to use
https://www.ibm.com/support/knowledgecenter/SSSA5P_20.1.0/ilog.odms.cplex.help/refcallablelibrary/mipapi/getnummipstarts.html
https://www.ibm.com/support/knowledgecenter/SSSA5P_20.1.0/ilog.odms.cplex.help/refcallablelibrary/mipapi/delmipstarts.html
to delete existing ones.