Redundant Default External Boundaries in FlexiblePatternGenerator

[miaoyinb]

Bug Description

A number of Reactor module MGs use a default external boundary id (10,000). Current FlexiblePatternGenerator does not delete this default external boundary from its input meshes. As a result, the output of FlexiblePatternGenerator would contain redundant default external boundaries that may cause error for further MGs.

Steps to Reproduce

[Mesh]
  [shg]
    type = SimpleHexagonGenerator
    hexagon_size = 1.0
    block_id = 100
    block_name = hexagon
    external_boundary_id  = 300
    external_boundary_name = 'external_side'
    element_type = QUAD
  []
  [fpg]
    type = FlexiblePatternGenerator
    inputs = 'shg'
    boundary_type = HEXAGON
    boundary_size = 3
    boundary_sectors = 1
    extra_positions = '0 0 0'
    extra_positions_mg_indices = '0'
    background_subdomain_id = 200
    background_subdomain_name = 'background'
  []
[]

The above input generates a mesh with two layers of external boundaries (id = 10,000)

Impact

Prevent users from performing complex patterning using the Reactor module tools.

[Optional] Diagnostics

No response