Fix invalid assignment to unallocated k_v/k_g arrays

k_v and k_g are allocatable arrays that are only allocated in
s_initialize_bubbles_model when bubbles_euler .and. .not. polytropic.
Assigning a scalar to them in s_assign_default_values_to_user_inputs,
which runs before allocation, is invalid Fortran and causes a runtime
error. Remove the premature array assignments; bub_pp%k_v/k_g (scalars)
are correctly initialized and the arrays are populated from them later
in s_initialize_bubble_vars after allocation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: sbryngelson

src/simulation/m_global_parameters.fpp

@@ -700,8 +700,8 @@ contains
         bub_pp%gam_g = dflt_real; gam_g = dflt_real
         bub_pp%M_v = dflt_real; M_v = dflt_real
         bub_pp%M_g = dflt_real; M_g = dflt_real
-        bub_pp%k_v = dflt_real; k_v = dflt_real
-        bub_pp%k_g = dflt_real; k_g = dflt_real
+        bub_pp%k_v = dflt_real
+        bub_pp%k_g = dflt_real
         bub_pp%cp_v = dflt_real; cp_v = dflt_real
         bub_pp%cp_g = dflt_real; cp_g = dflt_real
         bub_pp%R_v = dflt_real; R_v = dflt_real