Remove repeated value for the speed of light

[Oscar-PL]

Speed of light was defined twice

[jgalan]

It seems for the moment only lightSpeed is used in the repository

jgalan@sultan2:~/rest-framework/source$ grep -rnw . -e "lightSpeed"
./framework/tools/inc/TRestPhysics.h:56:const double lightSpeed = 2.99792458E8;
./libraries/axion/src/TRestAxionField.cxx:97:    Double_t tm = REST_Physics::lightSpeed / REST_Physics::naturalElectron * 1.0e-9;  // GeV
./libraries/axion/src/TRestAxionField.cxx:114:    Double_t tm = REST_Physics::lightSpeed / REST_Physics::naturalElectron * 1.0e-9;  // gev
jgalan@sultan2:~/rest-framework/source$ grep -rnw . -e "speedLight"
./framework/tools/inc/TRestPhysics.h:41:const double speedLight = 2.99792458E8;
jgalan@sultan2:~/rest-framework/source$ 

Should we keep then lightSpeed?

[jgalan]

Plz, approve #340 it is a fix to avoid undesired warnings.