remove unused themes

diff --git a/themes/digitalgarden b/themes/digitalgarden
deleted file mode 160000 (submodule)
index f087bcd..0000000
--- a/themes/digitalgarden
+++ /dev/null
@@ -1 +0,0 @@
-Subproject commit f087bcd10adc6ae4176dced9004283fee1507b57

diff --git a/themes/plain b/themes/plain
deleted file mode 160000 (submodule)
index 1c01094..0000000
--- a/themes/plain
+++ /dev/null
@@ -1 +0,0 @@
-Subproject commit 1c01094c67cf50dff0b140a672ba339c582ce2c5

diff --git a/themes/plain-hugo-theme b/themes/plain-hugo-theme
deleted file mode 160000 (submodule)
index 1c01094..0000000
--- a/themes/plain-hugo-theme
+++ /dev/null
@@ -1 +0,0 @@
-Subproject commit 1c01094c67cf50dff0b140a672ba339c582ce2c5

diff --git a/themes/simple-style b/themes/simple-style
deleted file mode 160000 (submodule)
index 19bb01a..0000000
--- a/themes/simple-style
+++ /dev/null
@@ -1 +0,0 @@
-Subproject commit 19bb01a6d805a05c9f8279c8d12d847e88129c85