Remove BlockMutex (#5521)

BlockMutex never worked reliably in production and has no users in the
codebase. With the increasing use of AI-assisted development, we want to
proactively remove unreliable synchronization primitives so that
generated code does not inadvertently adopt them.

Author: WeiqunZhang

Src/Base/AMReX_BlockMutex.H

@@ -10,8 +10,6 @@ foreach(D IN LISTS AMReX_SPACEDIM)
        AMReX_Algorithm.H
        AMReX_Any.H
        AMReX_Array.H
-       AMReX_BlockMutex.H
-       AMReX_BlockMutex.cpp
        AMReX_Enum.H
        AMReX_GpuComplex.H
        AMReX_Order.H

Src/Base/AMReX_BlockMutex.cpp

@@ -22,9 +22,6 @@ C$(AMREX_BASE)_headers += AMReX_GpuComplex.H
 C$(AMREX_BASE)_headers += AMReX_PODVector.H
 C$(AMREX_BASE)_sources += AMReX_PODVector.cpp
 
-C$(AMREX_BASE)_headers += AMReX_BlockMutex.H
-C$(AMREX_BASE)_sources += AMReX_BlockMutex.cpp
-
 C$(AMREX_BASE)_headers += AMReX_Stack.H
 
 C$(AMREX_BASE)_headers += AMReX_String.H