Remove unused broken GetTypecode to allow building with GCC 8

[rebecca-palmer]

This template function is unused (not in any public headers and not called from within clSPARSE), and has always been non-instantiable (and hence unusable) as char[4] to char& is an invalid conversion.

Older GCC didn't notice as we never try (instantiating a class template only instantiates those member functions that are used), but in GCC 8 its existence is an error:

/build/clsparse-0.10.2.0/src/library/io/mm-reader.cpp: In member function 'char& MatrixMarketReader::GetTypecode()':
/build/clsparse-0.10.2.0/src/library/io/mm-reader.cpp:123:16: error: invalid conversion from 'char*' to 'char' [-fpermissive]
return Typecode;
^~~~~~~~
/build/clsparse-0.10.2.0/src/library/io/mm-reader.cpp:123:16: error: cannot bind rvalue '(char)((char*)(&((MatrixMarketReader*)this)->MatrixMarketReader::Typecode))' to 'char&'

An alternative to removal is to change the char & to char *. Both options build in Debian (gcc 8.2, clsparse 0.10.2.0, where this problem was previously reported), but have not been built in develop or tested anywhere.

[rebecca-palmer]

The Appveyor log looks like a missing dependency before it even got to my change - given the long period without commits, does it now also fail on unmodified develop?

Scanning dependencies of target OpenCL
[ 3%] Building C object CMakeFiles/OpenCL.dir/icd.c.obj
icd.c
c:\projects\clsparse-otonj\bin\opencl\icd.h(53) : fatal error C1083: Cannot open include file: 'CL/cl.h': No such file or directory
NMAKE : fatal error U1077: 'C:\PROGRA2\MICROS3.0\VC\bin\amd64\cl.exe' : return code '0x2'
Stop.