Removed invalid assignment operator with const qualifier

In a personal project that depends from MRPT, I detected a problem in the libs/base/include/mrpt/math/CMatrixD.h file when compiling with clang compiler.  It throws the following error:

/usr/local/include/mrpt/base/include/mrpt/math/CMatrixD.h:77:12: error: binding of reference to type 'mrpt::math::CMatrixD' to a value of type 'const mrpt::math::CMatrixD' drops qualifiers [Semantic Issue]
                                return *this;

Actually the block of code removed is the same that the one appearing just some lines above (55-59) but with different typename and without the const qualifier.