Remove unused SliceDiagnostic.H/cpp (#5617)

The functions defined in `SliceDiagnostic.H` and implemented in `
SliceDiagnostic.cpp` are never used in WarpX. Therefore, this PR removes
these two source files (and updates `CMakeLists.txt` and `Make.package`
accordingly)

Author: lucafedeli88

Source/Diagnostics/CMakeLists.txt

@@ -7,7 +7,6 @@ foreach(D IN LISTS WarpX_DIMS)
         FullDiagnostics.cpp
         MultiDiagnostics.cpp
         ParticleIO.cpp
-        SliceDiagnostic.cpp
         WarpXIO.cpp
         WarpXOpenPMD.cpp
         BTDiagnostics.cpp

Source/Diagnostics/Make.package

@@ -4,7 +4,6 @@ CEXE_sources += FullDiagnostics.cpp
 CEXE_sources += WarpXIO.cpp
 CEXE_sources += ParticleIO.cpp
 CEXE_sources += FieldIO.cpp
-CEXE_sources += SliceDiagnostic.cpp
 CEXE_sources += BTDiagnostics.cpp
 CEXE_sources += BoundaryScrapingDiagnostics.cpp
 CEXE_sources += BTD_Plotfile_Header_Impl.cpp